I know its a terrible problem I have had the same trouble, what seat where you looking at. I got a great mothercare one, they wouldnt fit it for me either as I was putting it in the front (airbag turned off) they did sell it to me but they wouldnt fit it for me, in you case it didnt suit your car.
I found 2 rearward seats in Tony Kealys, if you go online you get loads…. Norway and Sweden keep kidsrearward until 4 years old…. my ds is nearly 10 months and I wouldnt even think of him going forward yet, 4 months is way too young if you had an accident your baby wouldnt be able to support the weight of her neck.
What weight is she? Have you moved the straps and removed all the extra padding?