you should have 36 or 37 gigs because of the number games that manufacutures play with their advertisment.
40 Gig hardrive = 40 * 1000 * 1000 * 1000 = 40 000 000 000
where as
40 GB = 40 * 1024 * 1024 * 1024 = 42949672960
So a REAL 40 gig hdd would be advertised as a 43 gig
As for the other 4 gb, i have noidea
