Re-tested with the booster hose blocked - engine running quite a bit better and the mixture screw on #4 is now responsive. Closing mixture completely on #4 only brings the RPMs to 800-900 though so I'm wondering if I in fact do have a little air leaking in elsewhere on the manifold, so I'll check for that tonight.
Any recommendations on the booster? I see new ones on classicalfa for about $250, and rebuild parts for about $100 but they appear to be for Benditalias. Or should I just pull the unit and inspect and go from there?