Keep it shorter. I am a 3 and I flip flop in the fields I race( I have been sick this year, so don't feel up to the 1-3 fields for longer races).

If the 3s want, they can do the 1-3 race and if it is a beginner 4 asking, then they should just try it for 30 mins.

I find, most people who have never raced don't understand how hard a race is, even though it is short. It is going to be 10x harder then training practice or a group ride,etc.

If it is longer- you run the risk of scaring away true beginners.