Allin has had a brilliant idea for simplifying drastically that area of the source, and possibly getting some efficiency gains too, but it'll take some time to implement in practice (and that's definitely something I don't feel confident about, so I don't think I can really be of any help).

That said, adding a fifth argument is defintely doable, so I'll see if I can get it done in the next few days. Testing what we have at the moment wouldn't be unwelcome, though (hint, hint).