Hello Nick,
The 520 should easily accomodate a 5th peer.
Found this - Although the 520 sh version may reflect IKE PEERS as UNLIMITED, the theorical maximum is of 2000 vpn tunnels. It is stated that with the VPN accelerator card optimum performance would be 100 Mbps; without the VPN accel card your encryption would be based on software thus significantly reducing the perfomance. Without the encryption card the 3DES throughput a PIX 520 is 20 Mbps at optimal conditions.
If you are absolutely sure the config is correct, try clearing Phase 1 and Phase SAs on both sides of the tunnel. This may resolve the issue.
Hope that helps! If so, please rate.
Thanks