Chris,
If you desire to upgrade from a Cat 4 to a Cat 3, you must race with others of similar category (Cat 4 in this case) or higher (Cat 3/2/1) in sufficient numbers to qualify the event for upgrade purposes, but you must also race and place sufficiently high with these similar category racers so that you earn points towards a requested (by you) upgrade based on these very same results. So, if you continue to race Master's, it is more of an age-based grouping rather than a specific capability-based, category-specific grouping. What this means is that when you race in Master's class, you may be racing with Cat 4, Cat 3, Cat 2, and sometimes even Cat 1 racers who prefer the camaraderie of "more experienced" racers to the pure contests within a Cat 4, Cat 3, or Cat 1/2 race event. So, in order to gain points in the Master's races, you would be racing head to head with some rather capable racers. This is possible, but you will need to bring your A game. We have some pretty heavy hitters in the local Master's races! We even get some heavy hitting racers from out of state sometimes and then things really can heat up. So, if you want to compete in the Master's while you seek to place highly enough to earn upgrade points, and given that you are a Cat 4 and since there are no racers lower than a Cat 4 in Master's, then you are up against some pretty long odds to get enough finish points to upgrade, especially since you have a time limit window (last 36 months from time of upgrade request) to earn and accumulate sufficient upgrade points.
What you should contemplate is racing in the Cat 4's, so that you have a better chance of achieving high enough results to earn points relatively quickly so that you can move up to a Cat 3 as soon as possible. If you are racing with the intent of gaining a higher Category, whether in CX, or Road, Track, or MTB, then the common wisdom is that you're not getting any younger, and racing against a more capability-based level-playing field like the Cat 4's would give you a better chance of placing highly than going up against a wide variety of levels in the Master's. The challenge will be that there is a wide variety of ages in the Cat 4's. You might be out there with racers from as young as 16 to as old as mid-50's or older.
So, you do have decision to make if you want to upgrade. The best strategy would be to really understand the USAC upgrade rules and work them as best you can to your advantage (same applies to anyone wanting to upgrade!) while enjoying your racing!
Cheers,
John Jordan, USAC Official