That's not what he asked cohen. He does not have an O.S. to install. He needs to copy the O.S. from the 4gig to the 40gig.
Do a Google search for MaxBlast software, download it, and then burn the image to a CD.
Set the jumper on the 40gig as slave and connect it to the second connector on the hard drive cable. The 4gig may need to be set as master and it will be connected to the last connector on the hard drive cable. Please be sure to have a newer 40pin/80 conductor hard drive cable to connect the hard drives with. Also be sure to connect power to each drive.
Boot to the MaxBlast CD (you may have to set the CD-ROM as first boot device in the BIOS) and click on the copy disk icon. Follow the prompts to copy from the 4gig to the 40gig hard drive.
When it's done, remove the CD, and shut down.
Remove the 4gig and replace it with the 40gig and set it's jumper to cable select.
Boot the PC.
You may have to reboot several times for it to settle down (driver installations).
Hope this helps.