It's a wee bit late to point this out now, but I personally would have kept the 4 GOS seperate for a while. Just to make sure they were all fit and healthy and not carrying any disease before putting them all in together. I'm not being funny - but have you read the codes of recommendations for keeping pigs ?
Under the biosecurity section mine says
"Incoming stock presents the greatest risk to the health of the herd as regards infectious disease. You should have isolation facilities so that you can isolate and test incoming stock for a period when they arrive, before they join the rest of the herd"It's from the Scottish executive, so I don't know if the English ones are different
Maybe a bit anal in some folks eyes - but I personally, wouldn't be taking any chances.