I would say its a bit long, although it depends on how familiar your LO is with your husband's parents and whether you have spent time away from him before. I think you have to make sure that both he and you would be comfortable.

We recently spend the first night away from our son, who is 13 months. I knew he would be ok as he knows my parents who he stayed with well, but I was a mess, which really surprised me! There is no way I would have coped with 4 nights! I think you need to work up to that, and if you haven't, then its too much for both of you.