I'm no engine expert, but I had the same question. I looked int it pretty well and the best I could come up with was that they are the same engine with different max RPM ratings so that the C4B5 produces a max of 250 hp. The purpose was to "de-rate" the HP for certain airframes. My understanding is this is a number thing only and the C4B5 is the same HP at the same RPM and fuel flow.
There are other threads on it here.
For what it t is worth, I bought the C4B5.
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Michael Delpier
RV6A -O-320, fixed pitch, GRT Sport, 496
RV-10 - working on finish kit
Houston
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