4U is hands down the best MH for newcomers trying to get into the series. It has the most well made single player "village" content and has excellent pacing all around which are areas that MHGen was weak in. Also, 4U does an infinitely better job than Gen at gradually introducing you to important mechanics in a transparent way.
Here's what I think you should do: Play 4U's single player campaign which Capcom actually put some effort into with the story, cutscenes and pacing, and then move onto MHXX for Switch when it comes out. MHGen and it's expansion MHXX really doesn't do a great job of explaining the basics but are worth playing due to the addition of super moves, weapon styles and the new super-boss "deviant" monsters.
