This was on my radar but then was suddenly out the other day I guess there's been so many games recently I lost track.
Anyway I had a go last night here are my thoughts.
Similar to JC3 you are on some island fighting an evil man with a private army called the black hand. They also have weather modification tech which is the new gimmick there are sand storms and massive lightning storms and tornadoes etc.
There were some performance issues initially cut scenes seem to have been rendered by a 486 in 16bit colour which is odd at one point the audio and video stuttered and then desynced so badly things were about 30 seconds offset. Not sure what is going wrong there not a good start. I'm running it on a fairly new machine with a rtx2080 and an i7-8700k so not cutting edge but pretty powerful setting on highest and 1440p res.
Once the game proper got going it felt familiar similar gameplay loop to jc2 and jc3 you destroy stuff it has missions there is lots of crazy explosions and vehicle and planes and so on.
Some new things the tether now has a new option that deploys balloons it has the same cable retract and rocket mods from jc3 now you can setup various quick sets with combinations of the tether retract, balloon deploy and rockets so that one of them triggers automatically when attached another when you tap the dpad up and then another when you hold the dpad up. So for instance you can have the cables retract automatically the balloons inflate on tap and then the rockets trigger on hold. The game has three presets you can configure as you like with different strengths and then modifiers that change the behaviour of all three selectable.
The world traversal is similar to JC3 you have the grapple and the parachute and then the wingsuit. The jc3 dlc modified booster wingsuit seems to be a dlc in this game too.
Weapons now have secondary modes some of them it's fairly standard like grenade launcher but other have things like a deployable shield lots of fun options although like JC3 if feels like you get very little ammo and I'm always running out and finding myself with no ammo. Since the explosives are now secondary modes on the guns you don't have a seperate grenade option.
Vehicle handling feels a little weird like theres some delay in the steering or something I find the cars difficult to control although like jc3 you only spend time in cars for the various stunts. There seem to be 3 types of sidequest the stunt series that includes the wing suit and vehicle challenges and are similar to jc3 but seem to have been dumbed down somehow. In JC3 the wingsuit courses were tough long runs with lots of targets to hit and required really precise control. This time they are usually 3 to 5 rings with a time limit it feels cut down. JC3 had races which I always found painfully hard JC4 now has speed challenges (move through a single gate at a given speed) and some where you just have to move a certain vehicle through a single gate ie a tractor or a specific bike or a boat or a helicopter or some such. That feels really reduced over the previous game. The next type of sidequests are the army training which are a mix of fairly standard missions hold a point against an enemy drive an set of allies somewhere escort quests etc. The third type is a sort of puzzle thing where you have to solve a set of physics puzzles using the grapples which is something a bit different.
The destruction missions where you had to destroy a base and get a certain score only using a certain gun or vehicle seem to have been removed. Which is a shame as I quite enjoyed those.
The game now has this mechanism where you advance the front in the war in the fictional island state. The chaos you cause by blowing things up allows you to recruit more troops and then as you can advance your frontline to allow you to access new regions and unlock new story missions. It's similar to JC3 with having to destroy bases to conquer regions and allow you to do story missions but you can't wander off and unlock unconnected regions it has to be part of the connected frontline of your war. There is also some sort of mechanism where you can surround a territory and make it easier or something not sure I understand that yet.
So if feels a lot like the previous games although some elements feel reduced or dumbed down a bit still has the crazy explosion destruction stuff. Some bugs some graphical glitches feels a little like they might have rushed some bits of it.