4-bolt stems make a lot of sense for a rough-use MTB. But, even for a strong road rider, I suspect that its not really necessary. Maybe if you are a track gorrilla it would give you piece of mind though.
The rest of us can happily use the cheapest $15 one in the bottom of the LBS drawer.
jim
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Cross Check Nexus7, IRO Mark V, Trek 620 Nexus7, Karate Monkey half fat, IRO Model 19 fixed, Amp Research B3, Surly 1x1 half fat fixed, and more...
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SB forever