Ok so we have OSPF back up and you have specific routes. You said there were no RE filters. Are there any firewall filters applied to the interfaces? Can you post the output of the following:
On R2:
show configuration interfaces ge-0/0/1
show arp interface ge-0/0/1.0
show route forwarding-table 172.27.0.6
On R4:
show configuration interfaces ge-0/0/0
show arp interface ge-0/0/0.0
show route forwarding-table 172.27.0.5