Here's a hot topic!
I did some number crunching with the crossresults.com points list.
Here's some basics:
For the Open Men:
3rd Quartile: 388
For the B Men (combined Cat 3 & Cat 3/4)
1st Quartile: 408
3rd Quartile: 466
For the Cat 4 Men
1st Quartile: 474
What does it all mean? I would suggest that any Cat 3 with lower than 388 points should seriously consider moving up. It would appear you could ride mid-pack in the Open class. Similarly, if you are Cat 4 with fewer than 466 points, it could be your time.
This would essentially shift 10-15 guys from the 3s to the Open race and shift 25-30 guys from the 4s to the 3s. This shift would change the fields sizes to: Open: 30-35, Cat 3: 40-45, Cat 4: 25-30. Seems reasonable.
Here's the full rankings: http://sites.google.com/site/kccrossrankings/home