I think you can buy manifolds for this purpose. Try ebay. I just googled and there are some already made.
https://www.google.com/search?q=yamaha+venture+4+carb+to+single+carb+manifold&rlz=1C1CHBF_enUS975US978&sourceid=chrome&ie=UTF-8
Its been done before.There should be some threads on this site someware on the process. i thought they was useing vw carb but i might be wrong. I think throttle response was the big problem.