1) Not unusual for a wimminz to be less frisky with her four year old in the next room.
2) If the sechuals are otherwise great, it's entirely possible the situation will resolve itself fairly soon.
3) Express your concern now, but make sure she knows you'll wait a little longer for her to readjust.
4) Revisit situation after a month or two and see if things get better.
5) Clandestine sechuals can be fun, especially if she's used to making noise. It kinda makes it sneaky.