If it's only been the last week or so, then yes I agree maybe she's got an ear infection or something. I would check it out. Other than that, it's possible she could be teething, although it's a little early, but not out of the question.
As far as the in law situation, I would get your husband to talk to them for you and stick up for you when/if your FIL makes comments.
I do feel like 4 months is a transition time for sure though. My kids sort of went from happy-go-lucky, sleeping anywhere babies to needing more structure, napping in their quiet cribs and setting a more consistent routine at this age. So, maybe it's just time to move into the next realm.
As far as your son, it's so hard. I would try to involve him as much as you can with the baby stuff, have him be your special helper, etc. Then find things you can do with him while holding the baby like maybe playing a game, coloring, taking him to the park, or even snuggling him while watching a show on TV.
Hang in there!