Hahaha! A little bug i just found:
If you want to connect to an older server using 1.4 client select the server you want to connect to, click "direct connect" then "cancel" and finally "join server". Enjoy
EDIT: Another solution (at least to when Minecraft 1.5 will be out): Make server send his version as 1.4, older clients will probably ignore it and connect anyway and 1.4 client will be able to connect too without using any tricks.
If you want to connect to an older server using 1.4 client select the server you want to connect to, click "direct connect" then "cancel" and finally "join server". Enjoy
EDIT: Another solution (at least to when Minecraft 1.5 will be out): Make server send his version as 1.4, older clients will probably ignore it and connect anyway and 1.4 client will be able to connect too without using any tricks.