gilbyrobb said:
On criterium days, do both races for a comparison.
If you have a velodrome in your area, go there first. They should offer, free training/drills on Saturday morning. Bring your pedals, they will rent you a fixed gear track bike.
This is the quickest place to learn how to race effectively. (This is my best and strongest advice)
Once you can marshall some pacing, drafting, sprinting skills, you can hang with the open Masters fields. (then you will improve your skills and fitness too)
You will not learn much about racing or tactics in Cat 4 as the races all seem to be anti-competitive, waiting until the big climb to thin out the field. In Masters, a break is always off the front, with blocking, bridging and chasing thrown down as needed. The finale is always fast--and from at least 500-1,000 meters out, not just the last 200 as in the 4/5s.
Generally, masters races are safer because the corner speed is much higher and the race is often single file due to the higher speeds. (not average speed, but a pursuit pace when the power is on)
You can have World & National Champions in Master racing.
Good luck!