4d vs. 6d question

How big are the differences between 4d and 6d? It seems like the percent difference is very small, what about differences in pen (ie, 66% pen on a 6d isn't really playable, but on a single deck it's very playable, how much more lucrative is a 75% 4d compared to a 75% 6d). Is there really a difference between the two from a cc perspective? Thanks for you advice.