I would go with the 4 pole xfer. A 120/208 generator system without a Xo bond could backfeed the line neutral looking for a return path to ground. This may be a safety issue if someone was working on the main service, with the generator running and open the neutral ahead of the bond to ground. The above post was VERY WISE about the problems with a GFI and a second ground. I was on a service call last year for that kind of problem. GFI trip as soon as the load exceeded the GFI trip settings. This took awhile to find due to size of the building.
