Where do you find the diagnostic port on vw golf tdi mk 4?

It is below the temperature controls/air con or climate control panel in the centre console.

If you look at the bottom egde of the upright part of the centre console, just above the ashtray, you will see a piece of wood grain or alloy trim (depending on your model) about 1/2" high and about 6" long. This covers the OBD diagnostics port.

Remove this by putting your fingers of one hand underneath and simply slide it back out toward you. Job done, To replace, just locate and push back in....simple as that.

The OBD port is to the RHS of the area which has been uncovered, and is usually a dusky shade of pink/light purple.

Hope this helps?