A multisignature transaction taking funds from #1 and paying them to #4 that requires signatures from #1, #2, and #3 should accomplish the same thing, though.
Great idea, Gavin!
So when we will have facilities to spend multi signature transactions without having all of the keys in one wallet?
Can you estimate how much time will it take to develop this?
I've been switching back and forth between writing a spec for it ( https://gist.github.com/2839617 ), implementing it ( https://github.com/gavinandresen/bitcoin-git/tree/signrawtx ), writing up a test plan ( https://secure.bettermeans.com/projects/4180/wiki/Raw_Transaction_RPC_Test_Plan ) and testing.
My goal is to get all that done by the end of this week and have it in the 0.7 release. But lots of things could make it take longer.