how to use general purpose registers apart from R4 and R5

There are documented ways to mix C + assembler, and generally you do need to either limit your register use, or preserve the values of any registers you may use and then return them to their previous values before any C routines execute.  Sometimes you may even have to prevent interrupts if you've disturbed the contents of certain global / compiler use variables, though this is a less common necessity.  Check the documentation for your compiler.

Typically there are also documents that show you how to call an assembly language function from C and in such a case you're more free to use whatever registers you want as long as you obey the calling and exit conventions relevant to your function call setup.

You can always write some C code and then compile it to assembler, inspect the assembler and use that as a guide to either refine the C code until it efficiently uses registers as you require, or you may choose to hand modify the generated assembler code and use it either as inline assembler or as a c-callable assembly function of your own customization to garner the desired performance / efficiency effects.

Typically it should be possible to get the C compiler to be as efficient as your hand coded assembly would be for most simple sections of code inolving loops or simple macros / trivial subroutines / inline functions et. al.