Like Eric stated, you need to add routes on both sides. You can do that by either having default routes pointing to each other via the /30 you're already using, or you can add routes for each host/subnet separately to go over the /30. You could add more dynamic routing over that serial connection, too, if you don't want to maintain static routes or use default routes.
For example, on R4, you could add:
ip route 0.0.0.0 0.0.0.0 126.96.36.199 and on RD you could add:
ip route 0.0.0.0 0.0.0.0 188.8.131.52.
As for whether all of the routers could reach each other (not just RD to/from R4), that depends on how your other routers route their traffic. If RD has valid paths to the other routers in the EIGRP AS and visa-versa, and R4 has valid paths to the other routers in the OSPF area(s) and visa-versa, and all of their subnets/hosts are part of the route statements that you set to traverse the serial interfaces via that /30 segment, then it should all work.
Edit 1: On RA, RB, and RC, you need to have RD be their next hop (which you can do as a static default route on each, if you just want the easiest way for now); on R1, R2, and R3 you need to have R4 be their next hop, which you can again add as a static default route on each if you want the easiest solution.