What operating system is it? If it's Win98SE make sure the 20gb has been Fdisk, partitioned and formatted. Install it as a slave to the 4gb. Boot the PC to windows and go to Start, Run and type in xcopy c:\*.* /e/h/k/r/c d: and press enter. When that has finished disconnect the 4gb drive and make the 20gb master and reboot.
If it is win xp download xxclone from click here and follow the instructions.