Not sure where you're getting your info, but they're not licensed for simulcast; there are no common frequencies between any of those sites. The only sites originally intended for simulcast were Columbus and Cleveland; as other counties joined they built out their own simulcast sites. Other counties not originally intended as simulcast have been or will shortly be converted to simulcast, but at the moment Darke doesn't appear to be one of them.
You're correct about the coverage though. Simply simulcasting those four sites wouldn't change coverage too much; the major difference would be reducing both roaming and the number of frequencies needed to cover the entire county.
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