As others have said, 4/6 matches common ISVs. Also, motor and mechinf platoons are deceptively fast on an actual mapsheet. Between Fast Movement, an easier time dealing with certain terrains, and the fact that they don't care about facing changes...a 4/6 Hoplite is probably going to have to dip into running MP on multiple occasions to keep up with a given 3 MP platoon. Unless you're specifically building something to support untransported foot troops, I'd probably say you shouldn't go below 4/6 at all, and that a 3/5 would have difficulty unless it jumps.
Finally, speed for infantry support units doesn't need to be slow, there are times when a fast unit is preferable. A Hoplite can keep up with troops and maybe maneuver a bit around them. On the other hand, a Hell's Horses force sees a Zephyros more than able to keep up, even able to pace some hover transports, and in a ground advance it's got the speed to quickly respond to events on the other end of the advance, while a Hoplite might be forced to plink with LRMs while it plods over.