Well, I think 4" vent is overkill. Let's see.
You have kitchen sink on its own vent, so that does not count.
You have 2 showers at 2 DFUs each
You have 2 toilets at 4 DFUs each
You have two handwash/lavatory sinks at 1 DFU each.
Total of 14 DFUs on this vent, going into 3" soil pipe.
Per table 916.1 of the UPC, you could vent that with 1-1/2" pipe if the total developed length was less than 32 feet. I'd choose 2" which allows 110 ft developed length and feels like more safety factor.
But you have an existing 4" vent - let's ask why rather than assume the prior plumber was a total idiot. The only reason I could think to use 4" would be in the attic and above the roof if having the vent freeze is an issue in your climate, so a larger diameter to resist freezing might be advisable there. In conditioned space, 2" will carry FAR more venting DFUs than you are applying to this branch. So you can probably skip messing with the wall framing and just bump it up above the top plate. The sort of weather that vents freeze in is exactly the sort of weather you don't want a plumbing problem in...
Likewise, unless you just wanted to stick with mostly 2" vent to save on getting various pipe/fitting sizes (and maintain overkill) you should be able to use 1-1/2" to vent the toilet/sink combinations to the stack.