I have replaced the screen on my iPhone 4s

The most common fault during an iPhone 4 and 4S screen repair is to damage the longest flex cable for the display. This flex cable is very fragile. This flex controls the touch screen digitiser. If it is damaged the touch screen will not work.

The most common way of damaging it is when the 2 display connectors are fed through the gap in the mainboard when fitting. Towards the base of the flex it doubles in width. It is at this point that it gets snagged on the adhesive on the mainboard and if forced a tiny bit further can tear slightly.

I would recommend removing the front completely and checking the longest flex very closely to inspect for damage.

If it is not damaged i would recommend checking that the flex is seated properly to the mainboard but i'd be surprised if it isn't damage to the flex cable.