Most cat 4 riders I know can hold 35mph for a while and do in most races, just a figure to aim for. Crits, well you certainly need good group riding technique there or you will almost be certain of being in an accident.
I don't know any 4th cat, 3rd cat, 2nd cat, 1st cat or elite riders who can hold 35mph on a standard road bike for any more than a few seconds - and sprint speed is largely irrelevant except for the final few metres of a race. The speed of a fast moving group is always significantly faster than the sum of its parts.
The conventional wisdom used to be that if you can ride 20 miles in about an hour, your fitness is roughly where it should be in order to ride competitively. Having said that, it does depend on the terrain you ride on. Round my way, even when I was race fit, I would struggle to average more than 17-18mph on training rides.
Don't be distracted by race average speeds. 3/4 crits usually run off at around 24-25mph average, but that doesn't mean you have to be able to ride at that speed on your own.