My iPhone 4 turns off all the time whatever battery percentage?

My iPhone 4 turns off every day, I took it to the apple shop and they said they screwed the battery back in properly as they said part of it was bent, but they didn't do anything else this was quite a while ago now it is constantly turning off and I have to press it in at the back for it to come back on, and when I release my finger it turns off.
Any suggestions as to what i could do?

Take it back to the shop and tell them to sort it