There are at least three driver varieties in the K400. First it's the hole (bass loading) at the back of the driver, in the center.
Then it's the amount of filtered openings under the ridged part of the diaphragm.
I'll describe them here as best as I can. I've had seven or eight K400 pass my hands. But it should be said that AKG made changes during production so use it only as a guideline.
1. Large driver hole with foam. This foam has deteriorated in all phones I've seen, and should be changed to new foam (or tape with a small hole, or micropore - it's possible to tune the bass here in many ways). Twelve filtered openings, all metal plug. Driver connections close together on the same side, with a blue glue blob to fix one of the wires to the diaphragm (which can cause an annoying rattle).
2. Large driver hole but with rubber grommet to make it smaller. Four filtered openings. May be possible to remove the grommet, but I've never tried - looks pretty tightly locked [EDIT August 2014: it can be removed, with a pair of pliers and some force]. All metal plug. Driver connections close together on the same side, with a blue glue blob to fix one of the wires to the diaphragm (which can cause an annoying rattle).
3. Small driver hole. Four filtered openings. Metal/plastic plug and leather headband. Driver connections on opposite sides of the driver (see red rings), like later K701. No blue glue.
If you take off the pads and hold the headphone in strong light (sun, if weather permits) you can easily see if it has four or twelve filtered openings, and if it's a grommet/small hole version.
If the centre of the driver is dark it's most likely the foam version, which probably needs to have its foam changed.
Also enclose a pic so you can see the difference between the plastic and leather headband versions.