Thanks Emma.
I am petrified of taking the dummy away because when she starts to get tired it really pacifies her and I can keep her up heaps longer during the day. I think I will give her to 5 months (she'll be 5 months old on the 20th) and see if things improve. Last night I had to replace her dummy 4 - 5times (and she settles in 5 seconds once I have)...but this broken sleep is KILLING me!!