If you have the Moldavy D&D Red Book...then you should have B4 tucked away in your collection somewhere...Moldavy wrote B4 himself for the Red Book so it's a bit of a no brainer. B4 is a sort of Tomb of Horrors for Original D&D, that is lots of traps but without the unbelievable fatality of S1 (yes we all love the Sphere of Annihilation...but this is an introductory module after all). My favorite element of B4 is that the wandering monsters are mostly made up of the wandering nutballs that inhabit the Lost City, which create a lot of opportunity for players to actually role play (i.e. ally themselves with various bands of city dwellers, etc.) and not just hack and slash their way through this dungeon.
B4 is a great module which may require more work on the DM's part than other Original D&D modules, so in a way it's an introductory module for players and a more intermediate one for DMs.
Long story short...if you love Moldavy style, then you probably own (or will own soon) B4 The Lost City.
[5 of 5 Stars!]