Same essence, sure, but MH4 is bringing many additions like platforming and villages.
Content is the biggest difference, we are talking about different generations, you won't find 90% of MH3U content in MH4 and vice versa.
I hope we do get MH4 in the end, because I'm really loving the MH3U demo and I'd love being able to play the "evolution" of the series. If my PS2 wasn't broken I might try to find the first game to check it out, I love discovering series like this and then going back in timeWell for one Tri U comes out next month and MH4 might not come to the West for a long time, if ever. (Has MH4 been confirmed to be localized? I don't even know.)
