Breaking down bike volume for more intensity

I think you could get away with your suggestion if you were already a fairly strong biker with HIM experience.  But otherwise, I would agree with Johnny that the average triathlete needs at least a few 3 hour rides for HIM prep.  They don't need to really extend to 4 hours (unless you plan to be on the course that long) but I think there is a pretty big drop off from the 1:30 hour range to the 3:00 range for most athletes.  If you don't get yourself used to it, you could have an epic blow up on race day.  More experienced cyclists don't drop off as much in that time frame, and also know how to pace themselves better because they've ridden 3-4 hours many...many times before.

How far out is the race? Far out and it might work out how you're thinking, but in the last couple months or so leading up you would want the larger rides in there. They don't have to be entirely Z2 either. Depending on how the overall schedule looks, I'd be putting in some harder than Z2 sections throughout the ride and also be looking at some hard work late in it. Both for the development aspects and to help test how worn down I was.