trip_to_tokyo is correct.
However, if you take the average size of your photos in MB and divide it into your 4GB (~4000MB) then it will give you a rough idea.
For example, if the average size of you photos is 4MB then,
No. of photos that'll fit on 4GB drive = 4000/4 =1000 (approx*)
For a more detailed explanation please see:
http://kb.sandisk.com/app/answers/detail/a_id/69/kw/how%20many%20photos .
It will probably be a little less as not all of the 4GB is available.