This is a tortured recommendation: Please read all the negative points I bring up (and consider some negative reviews) before coming to a conclusion of your own about this game.
The game has acceptable graphics, passable combat mechanics, basic spellcasting, and somewhat chaotic city management due to its region system. For those of you who played Planetfall, you will find that enough has changed to make this seem a completely different game. I will not be going over those differences in detail, but secret technologies are lost, race specific units and tech are lost (replaced by culture units without any tech), the positioning of your armies on the map does not affect their positioning in the battle (just two lines of armies go fight), and anyone can grab any tech provided they have first acquired the prerequisite techs.
Let's talk negatives, but we'll start with some things that some may see as a positive.
Unit size is very small: typically 3 to 6 models make up a unit. While many will view this as a vast improvement from games which show an entire company of soldiers as one guy on the map, this is still a downgrade from games which show hundreds of individuals making up a unit.
There is no uniqueness to any given play through's magic: barring your "class" which may have a paltry 1 to 4 spells unique to the class, any given playthrough can acquire any given magic.
There is no uniqueness to races: In my opinion, a fantasy game's biggest sin. In other words, races are just glorified, no, actual skins you apply to your faction. Your crocodile men are exactly like my humans who are exactly like my 10 foot horned oni. You can modify an entire race permanently with spells as the game goes on, but there is nothing bespoke here: it's the same limited spells everyone is going to pick and choose from gated by research. Cultist fans who have been brainwashed by Paradox will cry out "But you can select 5 points of modifiers to customize your race!" so I can make crocodiles who like the desert or humans who are poisonous, but these affect the game in extremely small ways and are mostly not even noticeable. Even seemingly major "adaption" traits like "can live in desolate areas" doesn't make a drastically different experience than a race of moles who prefer living under the sun in the jungle. They might as well be cosmetic.
You can stack enchantments on every unit of your army at once and without limit: If I want to give my elite unit of halberdiers an enchantment which causes the blades on their weapons to be aflame then I do it for every melee unit I own or will ever make. More egregious, if I want to then enchant those weapons to be frozen then the frozen weapons will still be on fire doing both types of damage. Some may like this, I despise this as massively immersion breaking. I'm just adding stats to my melee units: the fantasy isn't there.
Your ruler is just another hero which allows you to cast map spells while he's alive: You gain no benefit from parking your wizard king or dragon tyrant or whoever in your wizard's tower. This means that there is no great choice you have to make between the advantages of a parked ruler and a ruler out on the field using his power to defeat neutral and enemy armies. In short, if you're not constantly sending your ruler out to hoover up xp then you are handicapping yourself.
Naval "combat" has gone through many iterations, all of them terrible: ever since civ5 with its auto embark nonsense, the ocean has turned into nothing more than a "terrain where you go faster" zone. The current way the game handles naval combat is just your same troops in boats doing their same things. It's all shockingly bad.
Cavalry is just a movement bonus: instead of a mounted knight unit being a terrifying shock unit, the horse just gives him "very fast movement" which is about a 20% increase in speed. If I upgrade my cavalry through racial customization or in-game shop mechanics to use elephants, that just gives them +15 hp. It's absurd.
No unit customization: Don't like the units your culture has access to? Too bad, you're stuck with your 1 armor shock knights who look heavily armored in the pictures. For reference, heavy armor in this game is 9 to 10 armor. This is extremely annoying, especially late game where heroes.... actually let's talk about that.
Heroes just one shot things: this is especially bad when talking about rulers, but a somewhat leveled up hero can, on his own, just delete entire units who have achieved "legendary" experience. I hate the entire concept of heroes being on their own and not attached to units, but this just takes that complaint in brand new, uncomfortable, directions.
The AI: from being able to generate six stacks of armies every few turns to having endless legions of heroes outfitted with the most annoying and very best magical items, the developers have committed the, admittedly widespread in 4x land, sin of trying to make the ai fill the role of a player instead of be an element in the world. As a result, in order to present a "challenge," ai rulers have supercharged economies and the ability to produce entire formations of stacks out of thin air using said economy. It's certainly better than an AI who plays "by the rules" and is unable to get in your way at all, but it makes for a very tedious endgame as you must contend with unending legions of ai armies while you make sure not a single unit of yours dies so that you can continue to oppose said armies. This requires all sorts of cheese in the tactical battles which break immersion and, most importantly, make the battles stale.
The underground: I don't know what it is, but the underground feels like a slog to explore. They have made significant strides in making it more interesting with improved tunnel networks and the addition of points of interest, but it takes forever to explore the thing and doesn't really feel like an underground: just another overworld dimensionally linked to the other one. The other problem is that entrances to it are everywhere.
The worlds that get generated feel small, but are stupidly large in terms of traversal: it takes units an entire turn just to move across your average city's territory. Now include wilderness, pointless oceans, and all the 150 turn limit.
The game doesn't respect your time: My hours played at the writing of this review is 425 hours. That is mostly made up of the AI sending endless armies at me where if I hit "auto battle," my army would lose, but if I actually play out the battle then my army completely eviscerates the enemy without much thought on my part. I encourage the developers to focus more on large, climactic battles instead of endless nonsense ones.
So, you would think after all these problems that I wouldn't recommend it. No, on the contrary I recommend it in spite of all these things because this is where we are in 4x land at this time. If you want a modern fantasy 4x and you've already played Stardock's Fallen Enchantress to death, your options for something enjoyable are slim. Age of Wonders 4 accomplishes this in the same way that 15% nicotine gum accomplishes filling in the void for cigarettes. It's acceptable.