There are different part numbers for the pressure regulators on the 4 and 6. Anybody know what the difference is?

The difference is in the set point at which the "regulator" begins to proportion rear brake pressure.

914/6 has a lower pressure knee point due to larger rear caliper pistons 38mm vs. smaller 33mm pistons for /4.

Click to view attachment

I belive PMB can reset a /4 regulator to the /6 set point if need be.

There are also two types (early & late) for /4's where the inlet and outlet ports changed location to accomodate different mounting locations between early and late. If you were going to try to obtain and use a /4 regulator for a /6, you would want the early version.

Wow Superhawk996 that is great info again. This is just the best Porsche forum ever. Mark

Is this that part that is mounted in the engine bay, about 1/2 way up towards the driver's side with the brake lines hooked to it?

Yes

The previous owner had changed out the pressure regulator for an adjustable proportioning valve, I'm going back to stock. Are there any visible differences between the two, or do I have to hunt down a /6 part to have Eric rebuild?

I don’t think there is a visible difference. I think it’s more of a setting on the part to achieve a different pressure release. I would think it’s a needle in a haystack to find the original six part. I would ask Eric directly but I bet he can achieve the exact same performance from an early 914-4 piece.

Eric did write up a thread that dives into this part in detail.