Please don't include complete solutions in your posts. Please edit your post to remove the solutions to HalfAdder and FullAdder.
Once you have written and tested your HalfAdder.hdl and FullAdder.hdl you use them just like the built-in versions. As long as all three HDL files are in the same directory this happens automatically.
There is no way to embed the definition of HalfAdder and FullAdder within your Add4. You could include copies of their definitions as comments in your Add4.hdl:
/** Add4.hdl -- Four bit adder
Uses HalfAdder and FullAdder defined as follows:
[Cut and paste source for HalfAdder.hdl and FullAdder.hdl here.]
[Remainder of Add4.hdl.]