The A4000 may turn out to be problematic for you also in the long run. It's famous for having its capacitors fail (audio section and A3640 board) so be prepared for that costly scenario (trip to France or New Zealand if you aren't electronically inclined).
If you already have a working A4000 though, no sense in upgrading your A1200. Anyone that recommends that particular scenario is pretty goofy, drunk or high on something. lol
FWIW: Countzero, Fanscale & Tone's advice are very sound.
Since you have both, I'd make sure the battery is good/replaced/not leaking on the A4000. Sell your A1200 and purchase a Deneb card for your A4000. This makes the most economical sense IMO. A1200 is a money pit and you'll never be happy with it. Sorry sports fans, just my opinion. IF you hadn't of lost your 060 card, I'd speak differently - maybe. But I digress. Invest in your A4000, get her up and running and push her to her limits.
You already have a great Amiga foundation it sounds like!
Come to think of it, screw all my advice. Sell ME your A4000 for peanuts, since it's not really worth anything and go full bore with your A1200... :lol: