FormerUmpire,
You can find a downloadable PDF of the OBR rules here, courtesy of MLB:
http://mlb.mlb.com/mlb/downloads/y20...eballRules.pdf
I believe that any mitt or glove that conforms to the dimensional and construction constraints defined in 1.14 would be legal for an F4 - even if it "looked like" a 1st baseman's mitt.
Other might have a different opinion on the question.
You are correct that HS rules (explicitly) do not distinguish between a glove and mitt, and there is no difference in the constraints for what is legal for a first baseman vs. other fielders as there is in OBR.
JM