My dd is 4mths old too and she has a morning nap and an afternoon nap, either one long one of 2hours or two smaller ones.
I find that on days when she doesn't nap she gets very cranky!
How is she if you just lay her down in her cot for a while? Does she cry? I would maybe try putting her down after she has been awake for a couple of hours and see what happens. She may just be happy to lay there for a half hour or so. (try a mobile or musical cot toy) Get her used to having some quiet time if you can and she may eventually fall asleep.
Make sure you use the time to put your feet up!