I'm not quite sure what it is about Just Cause 4 that makes it so much less fun to me than the previous entries. JC3 was somewhat crummy, but I thoroughly enjoyed it. It was chaos, blissful chaos. Just Cause 4 still has the chaos, but it just isn't blissful. I'm not able to really enjoy it. Part of it, I think, is the pacing. I finished the intro, got a 2 second introduction to the map and mission system, and then was set free in an open world system that felt like every other sandbox in the world, but without any worthwhile indicator of success save for am occasionally moving red line.
Worse, though, was the combat, which was difficult enough that I spent the entirety of every fight in "near death" but never actually died. A thousand bullets, constantly at the brink of loosing, and I just didn't.
Then there is the fact that, when you start a fight, it pretty much continues indefinitely. I spent the first ten minutes of the game killing bad guys at the end of the first mission, before realizing that the fight would end. The next ten minutes were spent flying away from the fight to a chorus of notifications alternating between "we've lost sight of the target" and "found him, get him."
Ultimately, playing JC4 felt ultimately pointless and paceless, with only arbitrary objectives and victories to offset the monotony of endless violence. The chaos was just... less fun. It's hard to properly express that, but I hope this attempt was sufficient.