Okay, so I finally decided to do this.
In MH4U, I used Dual Blades vs Brachydios, with just me and my main palico Airha. Some things I noticed in no particular order: The graphics in MHGen look a LOT more refined, as do the menus and HUD. The backgrounds and textures are much clearer and more crisp and detailed in MHGen, even on the same maps. The character models, too. The palico AI is better in MHGen, as well (makes sense since they pretty much overhauled the whole palico system). I found myself fighting like I would as if I were using Adept style out of habit. Technically, everything is Guild style in 4, and honestly, even though I still like Adept better, Guild Dual Blades are still pretty damn good. However, I have to say combat in MHGen feels smoother and faster, and the inclusion of Hunter Arts really makes a difference. One thing I do like better in 4 is the demon mode and archdemon mode visual effects. Otherwise though, visual effects in general are better in MHGen.
I still like MHGen a lot more in almost all respects. Adept Dual Blades are my fave. And as much as I love my ace palico Airha, he's got nothing on my prowler Escandor. All that said, MH4U still holds up really, really, really well.