I actually vastly prefer EUIV to EUIII by a significant amount. The unfortunate wedding of Naval and Diplo is awkward, personally I feel like warships should have been on the Military tech tree, but in the end that's a quibble really. Everything else works for me a lot better. And it's nice to know that things will work, and usually why (even if I don't like the answer or think it makes sense) rather than magic 8 ball method of diplomacy -- results unclear, ask again.
What I do miss is the ability to found your own trade node. But the new trade system is so very much more intuitive, and monarch points at least make more sense than the "spend a guy" system it replaced. If only they hadn't included buildings in the MP system. That's the part that doesn't work for me.
If I had suggestions for the devs, this is what I'd put forward
1: SEVERELY curtail the role of dice rolls in combat. I'm sorry, there's just got to be a better way. Something based on a three way combat triangle would work a lot better. but the ideal is to allow you to direct the way your army is going to try to fight, in order to achieve objectives that make sense for the enemy you're facing and how you feel you should be trying to beat them.
For example, allowing you to choose tactical options to attack an opponent's Morale, maximize troop damage through attritional tactics, or minimize troop losses, or even fort up and dig in against a particularly aggressive foe, with different terrains and different troop compositions being better for different tactical options, would be a lot more strategic and tactical and would boost immersion. Even better if you COULD queue up orders for your troops, but it would be handled automatically and with some accuracy even if you didn't.
Just as an example, let's take the battle of agincourt. The French commander decided to use his numerical supperiority to rush his outnumbered foe with heavy frontal attacks and shock win on attrition. King Henry chose to fort up with a defensive infantry layout to blunt enemy shock and use his superior infantry fire and the forested terrain, to limit his own casualties and maximize his enemy's. The result is that French morale broke in light of the lopsided casualties and the English army won.
Right now the only way to even come close to emulating that kind of tactical depth is just to say that the Constable rolled a 0 and Henry rolled a 9 and had a few more fire pips. And seriously? I think in 2014 a strategy title can do better than that.
2: Naval combat needs help. Did you know that most naval engagements didn't result in the complete destruction of the enemy fleet? It's true. Furthermore, quality counts in naval battles -- a lot. A lot more than it does in the game at any rate. Look up the battle of Diu sometime. 18 portuguese heavies against 12 Indian tech heavies with about 80 light ships in support. That would have been not only a win for the Indian fleets in EUIV, it would have been a crushing defeat for Portugal with no survivors, even if you assume that the Portuguese fleet somehow did not manage to die of attrition before it even got to India in 1509. In the game being a generation or so back in naval tech is no big deal -- in real life, it meant that 20 ships could take down 200.
How I'd fix it is, ships that are a generation more advanced have a chance to outmaneuver opposing vessels and simply not take any damage at all in the round. That should be managed so that it's a decent chance if you're 1 generation ahead, a very high chance if you're 2 unit upgrades behind, and if you're more than 3 generations behind (not tech levels, generations, like if you have carracks and they have twodeckers) there's just no chance you'll do any real damage at all in any given round.
I'd also increase morale damage on both sides significantly so there's a somewhat better chance that the admiral who knows he's whipped will get his ships out of the fight and limp back to port.
I'd like to also make it possible so that, say, a 6 maneuver admiral could have a chance to get to sea past an enemy's fleet stack and avoid combat with a 1 maneuver admiral or an unled fleet, as long as they're not blockaded into a port. If your navy was free, you could do a lot of damage to a blockade, and bottling up the navy of another maritime power was an extreme undertaking. I wouldn't want to make it guaranteed even with a 5 or 6 pip advantage, but I'd want to make dealing with a maritime foe's primary navy as big a challenge as dealing with a terrestrial foe's primary army.
Blockading straits and short naval runs to unblockaded or neutral territory should take a lot more ships as well compared to blockading a standard coastline. A LOT more. Obvious examples being the English Channel, the strait between Skane and Sjelland, and the Bosporous. Basically if there's another unblockaded territory across a strait that would be considered bordering for the purpose of fabricating a claim, that should create a penalty to blockade efficiency.
Finally, there is such a thing in history as a siege from the sea. It should require Heavy Ships, and maybe even transports with infantry on them, but it should be possible to siege unguarded territory from the coastal waters, using heavies the same way you use artillery on land -- to simulate bombarding a fortress into submission. if your enemy doesn't like it they can drive you off the waters. If you don't want to go as far as being possible to force an outright surrender and transfer of territory, blockading with heavies should weaken the coastal forts above and beyond a standard blockade with other ship classes, so that if a stack does land and siege, it will come to find demoralized defenders and even a chance of broken walls.
3: The ability to meaningfully redirect trade. The upstream penalty is both far too big and far too bogus. it should be something that with the right combination of wits and technology, you can overcome or even reverse. 80% is just way, way too steep for that. Tone that down please, it'll make the trading minigame a great deal more interesting in the long run.