help!! Four year old just won't stay in bed!

I'm at my wits end.

My four year old dd goes to bed at 7.30pm and will sleep soundly until midnight, sometimes one am, sometimes two am. Then she will wake up and come into our room. Excuses range from I need a cuddle to you haven't cut my fingernails !!! I have been putting her back to bed three/4 times a night for the past three weeks now and I'm exhausted. Hubby gets up at five am every morning and does a fifteen hour day so it's normally all on me to get up and put her back down. 

Tonight has taken its toll and I find myself in her bed whilst she sleeps in with daddy.

I just don't know what else to do.

Any ideas??? 

Yours hopefully and exhaustedly!

Xx