Check out this web site: http://www.microsoft.com/technet/prodtechnol/windows2000serv/deploy/cookbook/cookintr.mspx
This should give you most of what you need to know. When our techs actually did our migration, they found 2003 has some wizards that automate most of the process.
If the 4.0 box can’t be cleaned up enough to have room for an upgrade to 2000, you have a problem. We had several boxes with 1 gig c: partitions, (don’t ask me why, it was done before I arrived). We did what we could to remove them from active directory but I still had to do some of the, (painful), process of manually removing a DC from active directory. This process exists for machines that die and leave a ghost in active directory. Avoid it if you can. See this link: http://support.microsoft.com/default.aspx?scid=kb;en-us;216498&Product=win2000