I have no opinion
Actually...
It really depends on where you race for example. Here in SoCal there is not a huge difference in many races, 4's race with 5's and 4's are not a huge amount faster than 5's (ask TCR for example), like Ontario series 45 minutes Cat 4/5. If you want to ever upgrade to 3 then becoming a 4 as fast as possable is a paramount importance - you need lots of races to upgrade to 3 (25 with 10 top 10's) if not by points.
Looking back on my experience, racing in mens catagories I will always been slower than the guys, it is mainly due to recovery, etc. My criterion is a bit different therefore in relation to many others ideas on this area. I beleive Cat 5 is for expeirence and 4's and above are where the true "competative" racing starts... fives are like holding your hand...
That said it will be several years until I race seriously again.
Mandy