Having 2 2GB sticks would be better I think as it would work in dual channel (http://www.hardwaresecrets.com/article/133)
However like you stated, there'd be no more slots for further expansion.
A) Get another stick of 4GB and have 8GB Dual Channel.
B) Return the 4GB stick and buy 2x 2GB and have 4GB in Dual Channel.
C) Live with one 4GB stick for now.
I can't foresee how much RAM we'll need in 4 years, but I'd predict 8 or 12GB would be standard however 4GB is plenty for while yet.