The topology is shown as below.
R1, R2 and R3 are connected via Frame Relay; all of them are in Area 0 of OSPF. On R2, redistribute the network segment of 10.1.20.0 into OSPF. R1 and R3 are ISIS neighbors of R4, repectively. On R1 and R3, OSPF and ISIS redistribute routes into each other, and after the convergence of routes, R4 is not able to reach 10.1.20.0.