Found a 1999 4gb HDD, what to do?

You have a new paperweight. Turn it in to a recyling center maybe?
Yeah, I have a few of them myself. Heck I have a old WD Caviar 340megabyte drive in the closet.
If I remember correctly, it has a read/write speed of about 4mbps, and a burst rate of about 18mbps....

Fujitsu drives in those days were not known for their reliability, put them down on the bench too hard and you have a paperweight, that could explain why it's still in it's box.
Would you believe that drive when new cost AU$469.00, I can buy 3 x 1TB for the same money now and get change.

if its still sealed you might be able to sell it as a mint condition antueque in another 10 years  

10 years? Nah probably 20!

I just found an old IBM 2160MB 4200rpm HDD...

Also got an old Pentium MMX laptop to go with it. =D
Destination... more paperqeights...