Is there comparison somewhere between 3.4 and 4.0? What i'm looking for some major feature differences. I know there is new responsive design and post editor. But i don't think these are the only things for better user experience of new ipb 4.

There are quite a few entries in the Company Blog aka 'News / Announcements Blog. Although staff have been concentrating on the product rather than writing entries. This is a good thing.

You may wish to quickly look at the entry listings and read any relevant. I was going to post a list but there are quite a lot, the titles of each entry will let you decide if the subject is an item you're interested in however.

There are many many improvements as its a complete and total rewrite.

Best thing to do is download RC2 install it on a local testing server and play around with it... along with looking over the information Andy recommended above.