How would your proposed 4.5+ division be any different from 5.0? Assuming that there is still a 'regular' 4.5 (which is I think what you're saying).
I mean as it is, 4.5s can go ahead and play 5.0 if they want.
I can think of a few reasons:
1. It would allow 5.5s to play.
2. 5.0 is already very thin, and why I think the main reason the sandbaggers play 4.5 and why some 5.0 players just don't play league at all is because:
-- There isn't a 5.0 division in that section
-- They don't know any of the 5.0 team players
-- They dislike the 5.0 players on those small number of teams
-- Because there are so few teams, they don't want to play the same team/opponent week in, week out. I have seen one case where there are just 2 teams in the division and they play each other like 5-8 times or something...that is no league and can't be too much fun for everyone involved.
-- The 5.0 teams that do exist are exclusive to paying club members.
-- The 5.0 clubs that do exist are too far away.
3. Why don't 4.5s just play up? Well you have to have the right mentality and maybe some players just don't have that when they see on paper "5.0 division". In their mind they automatically assume they are going to lose every match because everyone is rated higher than them. They don't want to be the worst player in the league. Well now that is says 4.5+, hey maybe there are a few guys this 4.5 believes he/she can beat, maybe now, he/she is okay with it. That insecurity is gone. Maybe the next 4.5 player now jumps on the band wagon...
4. Just by simple fact that there are far more 4.5s, the ability to build up the division to much larger numbers I think is better due to a larger network. 4.5s probably know 5.0s that weren't playing due to #2, maybe some are best friends with 5.5s waiting to play. Now maybe a team with a 5.5, 2x 5.0, and 6x 4.5s looks pretty good. Everyone enjoys each other's company, they have a decent shot to win any match, etc. Having better players back up a weaker player I think gives that player a lot more confidence.
If I bring up World Team Tennis as an example. I played a couple years ago in the '4.0+' division (there was a 4.0 division too), and here you had players ranging from 4.0 to 5.5. Current/former college players played, there were more teams than there were 5.0 USTA teams. Talent was spread out and not stacked on a single team simply because there were enough bodies to make more teams and players wanted to play on teams with their friends. Everyone knew what they were getting into. The 4.0s knew they could get train wrecked by a 5.5, but they also knew there were other 4.0s in the league in the same situation and they didn't feel alone anymore. They also knew they could run into that other 4.0 in singles or doubles (most likely doubles) and have a good competitive match. It was no longer a lose-lose situation.