How can I use iPhone screen hardware as a standard monitor?
The display in the iPhone 4S does not support a standard VGA or HDMI signal as input. This is true both of the iPhone 4S and cell phone displays in general.

Cell phone displays use a very different type of signal - usually MIPI DSI.

Some hobbyist have produced boards that convert HDMI to MIPI DSI which will work with the iPhone 4S screen:

https://hackaday.io/project/364-mipi-dsi-display-shieldhdmi-adapter

However, this is something I guess you would do for the fun of it or for learning. It doesn't make sense in economic terms compared to buying a used monitor.