Usually, I would recommend getting an extension speaker in a separate cab.

The only thing holding me back is the fact that the stock speaker is only 4 Ohms.

An extension speaker will usually be connected in parallel with the internal speaker. This would reduce the net load impedance to as little as 2 Ohms (if the external speaker is also a 4 Ohm unit). That may not be a problem, but I would want to check that out before spending any money.