Field help (F1) and input help (F4) are standard functions throughout the system. You therefore cannot assign any other function codes to the F1 or F4 keys.
There are various ways of making field and input help available to users. For example, you can either use ABAP Dictionary functions, or program your own help functions.
Another special way to display lists of values is to use a dropdown box.
Field Help Input Help Dropdown Boxes