Hi Jane, I also have a dd who is 4.5 months old. That's good that your son loves his Dad. I think it's because he's probably used to seeing you but only see's Dad a small part of the day. One of my friends who has an older child told me that she has the same thing, that they're so excited to see their Dad but if she has a day where she's at work quite a bit or her child isn't feeling well, they keep asking for their Mummy. I think you might find that your son has a stronger attachment to you but you just haven't seen it yet as haven't had to leave him just yet.
You poor thing, when it comes to in laws it's always tricky! Since having dd I've become a lot more assertive though. With mine I think it's good that they try & settle her sometimes but occasionally she does this different cry & no matter who has her I will grab her back without asking, saying something like 'you need your mummy'. I think if you're not comfortable with the in laws having your ds when he's crying, politely grab him back without asking. Your his mummy & whatever you says go's.
I'd also be proud if your ds is comfortable around other family. I love that dd is great around her Dad or our in laws as I know if I had to go back to work early or needed to go do something she wouldn't be distraught if one of them looked after her for me. I think you're doing a great job & should be vey proud of that