Well, since no one else has taken a stab...
My main concern would be comfort and accessibility. Having them too close together or too far apart could hinder your results. Think about how and where you're sitting and make sure they're within easy reach, but also not in the way. I think mine are about 11" apart, but that's for three rods. If it were legal to have four, I may narrow the gap a little., but not much.
Having proper spacing at the rod tip is much more important, IMO.