I spoke with someone today who had successfully upgraded their portal from DNN 2.1.2 >> 4..3.4 (only because 4.3.5 wasn't out)
He is an extremely experienced developer.
It took him one full day.
He said the trick was to upgrade from 2.1.2 >> 3.2.2 first, and not worry about the modules and how they look. I cannot state what third party modules were running prior to upgrading, but he was happy with the overall job, BUT IT DID TAKE AN EXPERIENCED DEVELOPER 1 FULL DAY - so it could take some time. He probably tested a few things first and dropped some installations along the way to use the 3.2.2 build.
I have never been happy with upgrading because I'm not a developer who can go in with scripts and fix things, and my own experience on a 2.1.2 site that had some modfications to the TAB id references back from a DNN1 installation proved to be a waste of three weeks.
I thought I would share this with you since if it took and experienced developer 1 full day - expect that it's a trial and error process and I hope the site was backed up.
If it was me attempting it - I'd be doing a few local ones, before trying on a live site.
Nina Meiers