Can of worms type of question. You don't need a prop valve with 4 wheel disc brakes since you don't have rear drums. The prop valve "blows off" the pressure to the rear drums so they don't lock up. Drum brakes are non-linear while disc brakes are linear which is why the rear drums need the blow off valve.
Now having said that, lots of people do put a prop valve in a 4 wheel disc setup so they can adjust the rear braking. But that only works if the car is set up with a rear bias to start with. If the car has a front bias on the brakes and you dial down the rears even further then all you are doing is killing the brake effectiveness.
Confused yet?
Bottom line nobody can tell you what to do since we have no idea what you're working on. To properly design a 4 wheel disc system you need to know the weight distribution of the car and the types of brakes and what the static proportioning is front to rear and bunch of stuff like that.
Yeah, I don't either and all of that just blew right over my head. But I have stock single piston calipers on the front with 11" discs with el budget disc on the rear. The master cylinder is stock style with two resivoir.
I haven't considered creating a bias and honestly I'm not sure I would since I'll really just be cruising.
This is helping though and maybe you can help me get them working ! But if I don't need the valve, can I plumb them front/front , rear/rear and get braking without the valve?
