Hi,
When i want to take a pic at minimum focus distance I cant choose f/4 (in manual or aperture priority). It is maximum 5.2. Is that normal? I thought f/4 would be at all distances if I choose so. Am I doing something wrong or is my lens faulty? I bought it new and hardly used it.
Yes, that is extremely normal --- and Required. As any lens is focused closer, its focal length becomes longer, and its maximum aperture fstop is reduced. Macro lenses map this, and show the new actual fstop number, the number you see. Regular lenses are affected too, but they don't show a changed fstop, and they do limit minimum focus distance to a larger distance, so that any fstop change is in small degree.
See
Nikon 200mm f/4 Macro
near bottom of page, the chart of maximum aperture with close focusing distance.
For the simple theoretical thin lens (single element lens), if focused at 1:1 magnification, the fstop is reduced two stops, f/4 would become f/8. However, the 200mm is internal focusing, and then tricks can be done to reduce this effect, f/5.3 in your case.
Realize the most important fact for macro work.... at very close focus (1:1 magnification), depth of field becomes almost zero, and it is normally very essential to stop down, way down, f/32 or more, to regain some slight depth of field.