I agree with Cory, with different external IPs then I don't think sync only would work.
If you had the same IP addresses for the VIPs could you create more of an active/active scenario across all 4 bigips? You could still have active/passive locally if you want i.e. traffic group 1 is active on bigip 1 and standby on bigip 2,3 and 4 with a Failover Order (TMOS 11.4 onwards) selecting the local bigip as the first in the list. If the whole DC went down then the traffic group would float to bigip 3 or 4 in the other DC.
See if this link could help Active-Active
Just a thought to see if it might help.
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