BOAC,
I agree with Saab Dastard, I would just ghost the 4Gb over to the 20Gb and leave the full 20Gb in one partition.
Then you should not have any problems with your current system. Also you just need to but the same jumper setup (Master or Slave) that you had on the 4Gb set on the 20Gb too and you are set. That way you do not need to change the Hard Drives around.
If you want to change your Hard Drives around, I would think about doing a fresh install of your OS's when you move the Drives to the order you wanted. I would suggest with the current prices of Hard Drives, look at getting one or two 80Gb Western Digital WD800JB Drives and put everything on them. It is a much better option than running all the older/slower/smaller Hard Drives.
Take Care,
Richard