R1 and R2 are propagating a redistributed (static) default route to R3 and R4. R3 and R4 now get each 2 default routes. Only one is installed in the eigrp topoloy database. This one goes to the main routing table (R3--> next hop R1, R4--> next hop R2).
If I am shutting down interface on R3, default route pointing to R1, R3 doesn't do anything with the default route arriving from R4. The same happens on R4.