if ur noticing what u think is ghosting at fast and fastest ,, thats almost certainly overshoot. Its about as annoying as ghosting. On fast and fastest ull have faster pixel response but transitions will tend to overshoot their intended color/brightness, causing the 'inverse' ghosting.
The 4ms speced response time could be anything, thats why it should be ignored in general when looking at monitor specs.
Chances are its a single measurement of a single color transition to a specific transition %, at a certain overdrive settings ..which though ,, we have no clue becouse the manufactures dont say.
As for what ur response time actually is, ull have to find a review that specifically tests pixel resposne, in the end though if u dont notice any ghosting on tis 'normal' settings, then, although it is likely ghosting, if u dont notice it all is well, and u really dont need to know the response time.
EDIT: since its VA (i think), chances are its ALOT more than 4ms, even at its fastest settings. VA's tend to be at fastest (top of the line models) 10ms, being a lower end model i'd expect to be 12ms + as a average of all pixel transition speeds.