We had a 3 year drought between Tri and MH3U. While I'm sure we won't have to wait as long this time, don't expect it until 2014 (I'd like to be wrong).
Release dates and times between upgraded versions:
MH 11.3.2004
MGG 20.1.2005
----------------------
316 days
MGG 20.1.2005
MHF 1.12.2005
------------------
316 days
MH2 16.2.2006
MHF2 22.2.2007
---------------------
372 days
MHF2 22.2.2007
MHFU 27.3.2008
-------------------
400 days
Tri 1.8.2009
3rd 1.12.2010
--------------------------
488 days
3rd 1.12.2010
3U 10.12.2011
-------------------------
375 days
The situation is a little bit different this time since the main entries usually started on consoles and then transitioned to handhelds, but we should still expect MH 4G around a year from 4's launch and I beat that's the version the West will get. Outside Japan the MH brand isn't nearly popular enough to be able to support two ''similar'' versions one after the other and Capcom won't the make the upgrade DLC because they want to sell full priced versions in Japan where handheld entries get most of their sales.
So I'm predicting we will get MH4G end of 2014/early 2015 if Capcom and Nintendo hurry up with the localization ;_;