Protector IV, the (semi)latest of the series.
Unlike the other Protectors, this one has the most changes, but even if they're the most, they're still not that significantly good.

Other than that, this may really be the best Protector there is, but like almost everything, it is subjective.
Personally, I think it IS the best, beats all the other Protectors, but not by too much, got some flaws I don't really like.

What's new?
- HEROES
Heroes had already appeared in Protector III, but these ones are now 'Generals' instead. You choose them to get units, you don't have your own basic units.
Each General/Hero is unique, more so than in Protector III, since experience depends on them now and they get each individually.
Generals are able to have an inventory now, which can give some bonuses to their skills.
You can choose up to 3 heroes on each level, any time.
- DYNAMIC QUESTS
In some way...
From now on, you can get quests that appear 'magically' out of nowhere.
These quests have different difficulties and the like. Sometimes you get an easy one, which is good for starters, sometimes a very hard one, bad for newbies but great for ol' veterans/longplayers.
I do like this system but I hate how it's unrelated to your character. You have low XP, you don't have many heroes, yet you get a quest that's 'Epic' in difficulty. What the hell, game?
Just give me a good quest for myself, only SOMETIMES give me a hard one if I want to do it hardcore.
Worst thing about it though, is the fact that your map can get fluttered with quests...
- GENERAL DEPENDENT ELEMENTALISM
You no longer have to train up your units just so they can get an Elemental on them.
This time, they depend on the general itself.
It can be a bit problematic, what if you need a fire mage instead of a fire warrior? You'll have to, NOT find, but BUY (hire) a hero/general for it.
It pretty much kills that annoying need to perk-up your units to get an elemental, but adds another annoying need of finding a general that has what you need.
- GENERAL DEPENDENT EXPERIENCE
Be wary this is not for UNITS, it's for the Generals. If you get a General's unit and make him get experience, that experience will also give it to the General. You can have all 3 generals have all their units in, although that can kill some experience gain, and who knows if you need them? A level with no enemies with weakness to Frost, you put one unit for it, then you start getting dumb at the fact they absorb that frost damage.
I am exaggerating though, you'd be very dumb to do something like that unless (but still dumb anyway) desperate or impatient.
Oh and, this isn't just for generals. You keep the experience system in Protector III, except a bit simplified so you don't grind too much for it.
For yourself, the skills will work on all units, whichever faction/element they are.
- NEW UNIT XP SYSTEM
Not new in the experience gain part, you still put them and make them attack, dealing damage which transforms to experience.
But they do have new SKILLS to choose.
Instead of making it be purely a tree of one (that is, you choose elemental, specialism and profession like in Protector II and III), it's a tree by itself.
For example, for a Warrior, you can choose 3 skills at first, when you get two you can reach the second skill (which depends in which trees you have chosen on the first) and then the third skill, which all are connected.
Nearly all Units have their own unique skill system. Paladins can get Consegration, a favourite specialization in all Protector games, while Mages & Trainees have the same skills.
Sadly, the XP system is a bit limited, it's capped. you can't get all the skills, but I suppose this is for balance reasons.
- INVENTORY
An inventory system has been put, a very shitty one for that though.
The inventory management is very petty, if you get a lot of it, expect it to be a huge problem.
Most of the inventory is simply loot that give skill bonuses. They can be useful, of course, but to me, it's a bit of a gimmick.
You'd be very lucky to find one that gives out some cool bonuses instead of just plain skill increases.
And with inventory, also comes a...
- SHOP
That's right. You can get into a store now, that has randomized placements of different inventory items.
You can also sell your own if you want to.
The shop generally has some very useful stuff, such as an order where you can change a hero's name, an order to change a hero's unit (randomly though, can be very shitty), etc, etc.
You can go to "Arkandia Jones" instead (yeah yeah, Indiana Jones reference, how original) and get him to go for epic items, but it's very costly.
- NEW UNITS
FINALLY! After three Protectors, there are, let me say yet again...FINALLY, new units!
You get some returning ones, such as the Paladin/Crusader and I think the Alchemist, but some others are left out, although that is compensated by the big variety of units.
Most of the units are unique in their own way, but the gameplay is not exactly too different from each, so don't expect some super changes.
Is that good or bad? Subjective. In my opinion, I like changes and these are good enough, but not something that makes me wanna replay the game about or so.
Also, remember how your units would change in Protector III as long as you level them up? Such as the warrior getting paulders and a helmet. Well, that has been removed, 'cos of the new units. Not really a bad thing per se.
- FACTIONS
Rather gimmicky and somewhat more decorational than anything...
Since you're now a Mercenary instead of some General to some lord or country, you pretty much have access to all the factions.
These factions are, as I said, somewhat gimmicky, at least to me.
I mean, luckily enough, they ARE different each and they do have their own uniqueness, but their gameplay in the game is almost the same.
Of course, each faction have their own units.
ALL Heroes are part of a faction, you start with that are part of Kingdom of Arkandia and one from the Council of Magi.

Also, there are faction relations.
The better relationship you have with a faction, the more damage their units will do, the cheaper their heroes and so on.
It's actually very good to be nice with these guys. Hell, you're a Mercenary, you're supposed to be professional and kind to all, even to bastard psychopaths as long as they pay you and treat you with respect.

And just in case you want to know if something returns, then sure!
- AWARDS
Awards, first seen in Protector III, are also in Protector IV! Renamed to "Talents" though.
There are way more rewards than in Protector III, and these ones are way better than in 3, because they aren't that hard to get anymore and they give (slightly) better rewards.
- MUSIC
This isn't something I like though.
Music from the old Protectors returns, but now, only FOUR are there.
Worse yet, they're exactly the same from before, by ADG3. Only one song is new and it's rather boring to listen to.
I really wish the author could do something about this, but instead, he only made it worse.

So, anyway...
In terms of gameplay, do expect a radical change, it's an even more radical change than Protector-to-Protector II, but you'll get used to it, I'm sure.
It's still the same anyway, you get two units (each unique, from each hero, with different elements depending on the hero) and you put them in, you make sure they deal damage to get XP so you upgrade them to be better and stronger.
Quests are also significantly different, they're semi-randomized and you can't really replay a quest from another unless it is on the Storyline track.
Don't expect the heroes to be like the ones in Protector III, you already know why.
I do find this to be a bit more fun than the old Protectors, but it's just virtually almost the same, so it gives no 'extra' bonuses or more fun.

Graphics have only been changed for the units. The landscapes and creatures are all exactly the same.
They're as smooth as in Protector 3, that is the animations and quality, but that's it.

For sound, guess what? It's all the same.
Same grunts, same creature noises, same everything. I already mentioned about the music, it's the same author with the same music, except 4 have been removed for virtually no reason (maybe the game had no more space or some such).

So, overall, if you are a fan of Tower Defense, this is the only Protector you should try first, even if there's I to III.
If you're a Protector fan, you MIGHT like it, but chances are you'll prefer III over all others.

Anyway, go ahead, try the game if you want, see if you like it or not.