These are early Playmobil railway waggons and so I'm not over familiar with them. However, I would say you should check for the following>
1. 4120 - check that
the handles on the end balconies and the red brake controls are there;
the red buffer stops are there, two at each end
inside all the seats are present, that there are two luggage racks, one down each side, and that there are two vertical poles
underneath the wheels run smoothly and bogies turn easily, that the bogies are firmly in place (there may be one or even two of the clips broken if someone has tried to take the bogies off the carriage, but so long as they are still fit firmly this need not be a problem) and that the "springs" on the coupling units are not broken.
4121 and 4122 - as for the 4210, apart from the inside checks, plus that
the walkway on the roof is there,
the securing hooks for the doors are there,
there is a set of steps each side.
I think that's it. Things most often missing seem to be:
on the 4120, seats and the two vertical poles;
on the 4121 and 4122, the walkway on the roof and the steps on the side.
