The 35+ 4/5 idea is probably not going to take away significantly from either the 35/45 1/2/3 or the 4/5's. I would resist the urge to further break this down, i.e. having a 45 or 50+ 4/5. The 35+ 4/5 is really a "4/5" race, that is one based mostly on ability. That is the most equitable way to segregate the fields, rather than having an age-based segregation.
To echo some earlier posts, I would be reluctant to tinker too much with the masters formula that seems to be working. This year there were actually 45+ fields large enough to deserve the term "field". In the not-so-good old days we would have 10 guys in the 35/45 field, which is not a road race, but a bunch of individual TTs mixed in with some negative tactics. Don't be afraid to let the fields grow above 50, that makes for a better race for everyone.
FYI the 45+ race I did at Chalk Creek was one of the sketchiest fields
I've seen, and it was only about 40 riders. Just because you're old
doesn't mean you know how to ride your bike. I would argue that that a
Cat 5 is a Cat 5. If you're going to have a separate 35+ Cat 4/5 field, then you should also designate the regular Masters as 1/2/3. It's no different than the regular 1/2 race, you want race with the "big boys", then get your upgrade.