On an NTFS 5.0—formatted drive (i.e., a Windows 2000 partition), you can't revert back to NTFS 4.0; you'll need to reformat the drive. In addition, you can't install Windows NT 4.0 directly onto a preinstalled Win2K system. When you try to install NT 4.0 on a preinstalled Win2K system, NT 4.0's setup reports that the existing partition is corrupt or damaged and asks you to format or repartition the drive. The NT 4.0 setup CD-ROM comes with Service Pack 1 (SP1), which doesn't recognize Win2K's NTFS 5.0 (NTFS5). Only NT 4.0 with SP4 or later recognizes Win2K's NTFS5 partition.
To install NT 4.0 on a preinstalled Win2K system without formatting or reinstalling Win2K, you need to install NT 4.0 SP6 on a spare PC that has SP4 or later—preferably SP6. (SP6 is preferable because SP6 supports a version of NTFS that's compatible with most of Win2K's file-system features.) Then, use the original version of NT to copy the new NT folder to the preinstalled Win2K system and modify the preinstalled Win2K system's boot.ini file accordingly.
Let's assume you have two PCs: PC1 has Win2K loaded, and PC2 has NT 4.0 loaded. To install NT 4.0 on PC1, first install NT 4.0 SP6 as a second OS on PC2 in the \nt40 folder. Boot to PC2's first NT OS, and copy the \nt40 folder to PC1. Add the following line to PC1's boot.ini file:
multi(0)disk(0)rdisk(0)partition(1)NT40="Win nt4.0 \[VGA mode\]" /basevideo /sos
You'll then be able to dual boot on PC1 between Win2K and NT 4.0 on a Win2K partition. You can remove the second NT installation on PC2, which you used only to prepare the NT 4.0 SP6 boot file folder.