First off, don't be so quick to count out the 6 bar as an option. It may not be as effective as a rd4b, but that doesn't mean its a bad choice, especially considering you seem to have trouble building a rd4b. It's never a bad idea to try new things and innovate options that may not be popular, after all, that's how the greatest ideas are formed. But if you would like to go with a rd4b, there are plenty of pictures and videos of them in effect, so you can just use a motor configuration that has worked for others. 4 motors are more than enough, so you've got room to try pretty much whatever you'd like.