Well I say fourth, actually I only have three now, as I gave one to Gareth as a birthday present a few years back.
One pair is on long term loan to Dave the Tree, and another has both bass units dead in one speaker thanks to a lightning strike last summer causing my power amp to throw DC at it. So I will have one and a half working pairs at my disposal as soon as the new ones are delivered on Saturday.
I plan to either get two new drivers (95 quid + VAT each) to fix the half-pair, or cut the boxes down to a more appropriate size for a single driver. Or maybe they want to be fitted in the car somewhere. Hmmmm.