I am putting together a 4 cylinder vintage race car a friend who passed 5 years ago had started. I'm getting calipers from PMB, going with stock brakes for now. How many are running the stock brake pressure regulator in their race/track cars? Should I remove it and put in a proportioning valve?

That would be my recommendation. I prefer the lever-type so I can reach down and "feel" where it is without looking. With the rotary knobs you can't know without moving the knob to one extreme and then turning back.

https://www.summitracing.com/parts/til-90-1003/overview/

With stock brakes I'm not sure I would do it. In my case I have one, with 911 Carrera brakes, but it stays full open all the time and I just as well could have a 'tee'. I've left it in the car mainly in case of rain where I figure I may at some point want to dial-down the rears, and also cuz changing any of this stuff would be a PITA.

Under the advice of Mr Shea at PMB I am using it on my track car and it works great. I have the Alfa Romeo front calipers and Ferrari 308/ 914-6 gt rears. Remember this really isn't a proportioning valve. It is set up for panic situations to keep the rears from locking first. There are threads somewhere in this forum about this topic. I say leave it for now if you brakes are stock.

Correct that that it's not a true proportioning valve but rather a pressure limiting valve. In the stock '74 914 that I had years ago I could hear it actuating (when engine off) if I pressed hard on the brake pedal. It must be a spring loaded mechanism. Putting one of the racing type limiting valves like Chris recommended above will give you a firmer pedal, which is worth something for track use. You can also adjust it to limit the rear braking a little less which will improve braking performance for track use.

Actually, it is better to increase braking pressure to the rear in wet conditions.
Reduced overall grip from the wet means less weight transfer to the front is possible, thereby allowing the rear brakes to do a greater proportion of the work.

I am an advocate for adjustable brake bias in any race car. If you don't have it you are leaving time on the table. If you have it but don't know how to use it, you need more practice with threshold braking.

^^^

Many think otherwise because we were fed a steady diet of Trans Am and Indy cars spinning in the rain, with talking heads commenting about how they had too much rear brake bias. Which is true; it's a fine line between hero and zero...