Question: Can the German side place one of their own charges in the "4" neutral objective marker hex first that they may later detonate it to cause casualties to any U.S. squad that enters the hex attempting to place their own charge in the same to destroy the gun emplacement there? or not?

Also, the victory conditions mention: "The Americans win by eliminating the German units by the end of round 4". Question: Since there are 13 German squads, is it correct to assume this means eliminating ALL 13 squads (this seems very unlikely)? or, is it conceivable that this is a misprint and means some other "magic" number of squads (or, perhaps one or the other of all the squads of one or the other of the two german divisions)? or, that this is possibly completely incorrect and should otherwise be ignored?

Yes, you should be able to place a charge there to detonate it later as most squads would place the charge and vacate the hex to avoid damaging themselves when it was detonated. No reason you couldn't booby trap the hex.

And yes, you need to knock out all of the German squads. Which actually could be possible if the Americans focus on knocking off the German squads out in the open first and then go after the pillboxes with some assaults.