On all routers except R4


ip pim rp-address 180.40.7.129 1
!
access-list 1 deny   224.20.68.1
access-list 1 permit any


On R3 to support R4:

R3#sh run int fa 0/1
Building configuration...

Current configuration : 142 bytes
!
interface FastEthernet0/1
 ip address 180.40.7.129 255.255.255.224
 ip pim dense-mode proxy-register list 100
 duplex auto
 speed auto
end