Olly-hi and welcome btw, don't mean to be rude but do you really know whats gone on whilst you were away? Also i agree with Wendy in that these stray cats may have scared your cats and so they are no longer sure of themselves and of their home and are probably a bit confused/frightened so know you do but really gut your house from top to bottom and with your 4 cats just go slowly and gently in reassuring them that you are back and everything is as before