M.S.
Hi A.,
I have to tell you that my daughter was exactly the same way... She would only sleep in her swing at that time or in the carseat. For that reason, I would usually go grocery shopping or running erands when it is time for her to nap so she could get some sleep. I believe the problem is that at that stage (around 4 mths) they become aware of all these sounds and they wake up as soon as they hear something... I know it's hard but it will pass soon :)
Good luck!
M.