Each channel will see 4 Ohms, which is fine.
A couple of suggestions: (1) If you're currently using the PR15s on stands, keep doing so rather than setting them on top of your new subs. Running a sub on each side invites "power alley" (excessive bottom in some parts of the room, and none in others, due to overlapping sound waves). Instead, try to couple them (stacked or side-by-side), centered in front of the stage (some folks even use this as a stage extension of sorts, putting the lead singer's wedge on the subs). If centering them is not an option, coupling them on one side or the other still gives better results than having one per side. (2) Try to keep your crossover setting around 90-100Hz if possible, especially if your subs are on one side. If you go much higher than 100 Hz the subs' location becomes sonically apparent.
Hopefully you'll have time to experiment with various configurations and settings to find out what works best for you. Good luck!