Provided that there is no other conductive path between the house & the shop that would require bonding (thereby creating a parallel path), then you do not need a 4 wire feed to the shop & you do not need to separate the neutrals & equipment grounds. Take a look at 250.32(B)(2) and see if you think this might apply to this installation.
Your thought of running an equipment ground over from the house to the shop might be OK if you were dealing with DC circuits, but it would be useless for AC and a violation of 300.3(B) Soares Book on Grounding covers this subject really well & is worth the $45 or $50 investment.
Parallel conductors would have both ends joined together, since you're running to two separate structures, then 310.4 would not apply to the feeders unless you are actually running parallel conductors to each building.
Tom
