There are not many PS people at CW so let us be kind:
I modified the signature from:

function prod-2-sum($a, $b, $c, $d)

to

function prod-2-sum([int]$a, [int]$b, [int]$c, [int]$d)

.

In PowerShell I had to cast the parameters as [int], otherwise when they were passed as negative numbers they were being treated as string

Who treated them as strings? Negative numbers are [int] and they were passed as negative numbers; frankly I don't understand your problem.

You should mention the language you're having that problem. Those inputs seem weird.