Pretty much all masters races here in Arizona require Cat4, but that of course isn't universally true. There may be some experienced C4 masters in your race, which could help smooth things out a bit. Small field size helps, but less so in a RR. Racing mostly Masters, the biggest issue I have with the 4-5 races when I do them is how 'surgy' they are in the corners. There is a lot more braking and acceleration in a 4-5 crit, which creates a different feel to the pack, and I frankly have difficulty getting in sync with the pack. I dropped off the back of one awhile back because I felt so out of sync with the kids that I was dangerous, and the pack was large and difficult to get around to the front.