Where the arguments live
The GameCube ABI hands the first eight integer (or pointer) arguments to a function in registers r3, r4, r5, r6, r7, r8, r9, r10 — in that order. The first argument is in r3, the second in r4, and so on.
Consider a function third(int a, int b, int c, int d) that returns its third argument. Three out of four parameters are unused, so the only instruction is a single mr to copy the right register into the return register:
mr r3, r5 # r3 = the 3rd argument
blr
mr ("move register") copies one GPR to another. The three unused arguments arrive in registers but generate no code. Knowing the argument-to-register mapping by heart lets you read any function signature straight off its first few instructions.
Now look at the assembly for fourth. The mr copies a different source register — figure out which argument position that register corresponds to and return it.
Your task
Write fourth, taking four ints and returning the one whose register matches the assembly above.