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 0.0.0.0 202.20.0.1 and on RD you could add: ip route 0.0.0.0 0.0.0.0 202.20.0.2.

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.

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 0.0.0.0 202.20.0.1 and on RD you could add: ip route 0.0.0.0 0.0.0.0 202.20.0.2.

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.

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 0.0.0.0 202.20.0.1

  and on RD you could add: ip route 0.0.0.0 0.0.0.0 202.20.0.2.

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 0.0.0.0 202.20.0.1 and on RD you could add: ip route 0.0.0.0 0.0.0.0 202.20.0.2.

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.