I'm a cat5 this year (racing age 43), and I usually enter the master's 4/5 race if it's offered. Maybe it's a maturity thing, but the master's cat 4/5 races usually seem to be a little less dangerous/chaotic. Only time I might prefer an open 4/5 or 5-only race is if the race length is different from the master's (one race I did a few weeks ago, the open 4/5 race was 42 miles versus 32 for the master's, for example).