Cutting a blind keyway

[Highpower]

Everything I have read says to use a 2 flute (only) end mill for slotting / cutting key ways etc., so that is what I have been following. The reason given was that it keeps the tool pressure equal on both sides of the slot. I'll buy that for a dollar. I can see where 3 flutes would flex back and forth between the sides because of the offset cutting contacts, but what is the problem with using 4 flutes?  

Something I've always wondered about, but never thought to question WHY?