In the OmniWeb mailing list, someone on the OmniWeb development team said that they have been working on 4.0.x and 4.1 in parallel, with the main intent for 4.0.x to be bug fixes and 4.1 to be new features. He also said that they'll be releasing 4.1 around the same release time as OS X.I, although I seem to remember he was implying sometime slightly afterward. It'll be like Christmas in September.
4.0.x is mostly added languages... fixes for urgent bugs discovered/solved during 4.1 development and during localization process are added as well. You'll have to wait for 4.1 to see significant changes.
Omni web is nice...I just wish it weren't so damn slow. It's slower than IE in X and IE is reeeaaall slow. I'm hoping this will all go away with 10.1. I just tried running IE 5 classic in OSX and I couldn't belive how fast it loaded pages...totally amazing.