i'm using d90. when i open my popapflash the shutter can only go until 200. but my friend can go untill 4000. izzit setting or my d90 have a problem?

pop up flash can only support sync speed up until 1/200..
if u're using external flash then 1/4000 will be okay if the flash is powerful enough..

u guy know that light travels very fast right?
but after 1/250 the light wont be sufficient to reach the subject, esp when ur subject is far (when using pop up flash)
by getting an external flash, u can get power.. the flash will add more power to the flash/light that will make sure the light reaches the subject to catch up with the 1/4000 shutter..

but not all external flash can do it...
in nikon, its know as FP, its automatically enabled in sb600, sb700 and sb900.. other model ie sb30, sb400 i havent check yet..

thatis the logical part of the answer..
for the more 'technologycal' part u have to wait for someone else expert to explain..