If I understand you correctly you want to take a three pin fan connector
 the kind that attaches to the MB) and make it fit a four pin molex
connector. This is a job for weasel techs. All you do is splice the red to
red and the black to black and the white is usually ground. Make sure you
pull the metal out of the plastic housing first and wrap the wire around the
connector very well. This also works well for those annoying cpu fans that
want to plug in the board, that your board doesn't support. And last but not
least, you can repair all those old power supplies with the burnt-out fan in
a similar fashion.

Nothing broken  --Everything is fixable.

-AAAunderground