I use two flute end mills when cutting a slot, when both sides are cut at one pass.
Four flute cutters under the same conditions will 'walk' sideways, when one flute is in the lead, it's pushing the flute behind it into the side wall, so it will not cut to size. I use a four flute end mill to cut on one side only.
Having said that, when cutting a slot to a specific width I use a two flute smaller than the slot and make finish passes to get the slot to size.