mIRC scripting doesn't use quoted strings (with a few exceptions); to check if a value is empty compare it with $null, ie., $4 != $null.
That said, the way mIRC's tokenization works (the way it fills $1, $2, etc.) prevents empty tokens from being allocated. So if $4 != $null then $1, $2, and $3 cannot possibly be $null either - therefore there is no need to check those.
