I used my Dolmar 7300 with the 24" bar on a 24" oak last week, since you mentioned oak, although it wasn't 4 feet. The saw does fine. It's no problem to cut oak with this saw, and if you put it into anything less than a good size oak, it has to be held back so it it doesn't drop right through and hit the ground. It needs no coaxing, and the weight isn't that bad.
The dealer had a 7300 but no 7900 and didn't figure he'd be getting any, so based on price and availability, and the high cost of the Stihl saws I went with the 7300. It was $660 two years ago. I'm not a pro, but needed a big saw for those times when you need a big saw. I hope I didn't overpay. The 7900 is probably a lot nicer, and the Stihls and others are probably great, too. Northern New Jersey is not lumber country, and availability is scarce, so all that has to factor in when looking for a saw.
I've seen on Craig's list where some people completely rebuild saws and sell them. The nice Stihls and Huskys. There was even a big, hardly used Stihl. Lots of choices. And so little time....