Why is my 4yr old behaving like this?

my 4yr old has recently turned into a real whingey and horrible to deal with child - for me and his father. with anyone one else who looks after him he is as good as gold his preschool havent a bad word to say about hi. yet when he is with us he is rude, back chatty atiitude slams doors and throws things around the house at the simplest requests. i am really struggling to cope and feel exremly exhausted by the situation. im starting to feel guilty that its because myself and my husband work full time and he is with a childminder and his grandparents alot. iv only been back at work a few months and have tried to be the best mother i can be. his father and i recently seperated for a few months but were are back together now and everything is better than ever. i understand this may have unsettled him. how do we get him past this and why is it only us that sees this side of him. any advice gratefully received. we have tried the step, taking toys away even lightly smacking him which i really dont approve of but felt it was an option we could try but nothing works. im starting to really worry about it.