I'm running Mac OS 10.7.4 on an iPhone 4S and iPad 2 with iOS 5.1.1
The upgrade from YNAB 3 to 4 on the MacBook Pro was uneventful. The upgrade on iPhone and iPad is not working at all. Both devices report the same message "Whoops! There was an error downloading all of your budget data. Please make sure you are connected to the internet and try again." Seriously, we're blaming this on an internet connection??
I've tried to setup phone and pad multiple times with same result. When I look at the Drop Box account during the sync you can see that it is taking place and DB reports so with a message that device x has received the latest data, etc. But, alas, nada.