a0 through a3, in order
You've been living on one or two arguments so far. The o32 calling convention — the contract every function in this game obeys — hands a function its first four word-sized arguments in registers, in declaration order: a0, a1, a2, a3. No loads, no setup; they're just there when the function starts.
Here's blend4(a, b, c, d), which computes (a + b) - (c + d):
addu t6, a0, a1 # a + b
subu t7, t6, a2 # (a + b) - c
subu v0, t7, a3 # ((a + b) - c) - d
jr ra
nop
All four registers show up exactly once, in order. But look closer: the C said "subtract the sum c + d", and the assembly never computes that sum. IDO re-associated the expression into one left-to-right chain — subtracting a sum is the same as subtracting each piece, so it folds one argument in per instruction and never needs a second temporary chain.
That's the general reading technique for these: don't try to match parentheses. Walk the chain line by line, note whether each argument is added or subtracted, and write the flattened expression. Any C spelling with the same meaning compiles to the same chain.
Your task
Write func_8005a480 to reproduce the chain in the target assembly.