Hmmmm. It's weird because you're receiving the OSPF hellos on R4, and the ARP request from R4 is making it to R2. So you have a shared LAN segment, but R4 is not receiving the ARP reply from R2. Can you show the output of the following while outside of configuration mode:
(R4)
show interfaces ge-0/0/0 extensive
(R2)
show interfaces ge-0/0/1 extensive
Also, try sending a continuous ping from R2 to R4, and do "monitor interface ge-0/0/0" on R4.