i am cooking Prime Rib for fathers day and I purchased 4 - 4 rib Prime Rib each weighing approximately 9lbs each. If I put all 4 in the oven how long

Unless you have an enormous oven, I don't know if enough air would be circulating to roast evenly. Please do report back and let us know how it worked out.
Happy Father's day to alk the Fathers!

In addition to rotating the roasts, I'd make sure there's room for air to circulate around the pieces

Sounds like an old arithmetic problem.
But it's not exactly multiplying the weight by the # of pieces an then estimating how long one piece of meat that size would take to cook.
Yes, it will take longer to cook all 4 than just 1, and you may want to rotate the pieces halfway through (to avoid one piece getting a hot or cool spot).
But your best bet is to use a thermometer and remove meat from the oven when it reaches 120F for medium rare (or adjust up or down for other desired doneness).
http://www.geniuskitchen.com/recipe/perfect-medium-rare-prime-rib-roast-246601

Good question, but you need to specify degree of doneness, temp. of oven, temp. of meat to start etc. Thirty six pounds of meat is going to definitely throw things out of whack in a normal home oven, but someone will probably come along who's done it.