you "should" get the matching 4x10, but (assuming it's even the right impedance to work with your gear) if this 2x12 is any good it might be a little better match for your existing 4x10 than the 1x15 is. not great, but maybe a little better.
the question becomes whether spending a little cash now to get maybe a little better setup is as smart as using what you have and holding out 'til you can score the ideal cab. i gotta think the 4x10 on 1x15 is at least loud enough to gig with for now, right?
of course you might hear that 2x12 and decide you like it better than either cab you have now, in which case you should get it and use it with one of your cabs until you can get a second one and ditch everything else.
Guitar Repair Gnome
Alpha Music, VA Beach