But I am guessing you mean the feed rate

Both the feed rate and the spindle speed need to be adjusted when changing from 2 to 4 flute, especially when dealing with plastics.

 

Fast or slow speed? I assume slow? Same for MDF?

 

Fast or slow speed? I assume slow? Same for MDF?

A little slower, yes... same for MDF... the thing is that even though the tool has 4 cutting edges, their "throats" are not as deep and aren't able to remove and expel the material as fast as a 2 flute tool would.