NT4-SP4
For those of us waiting for a NT UDMA solution from Microsoft the release of service pack 4 came as a let down.  While the atapi driver has, in fact, been updated, there is still no support for bus mastering included in the service pack.

On a positive note, the idefix-i and atapi-fix post SP3 updates are included in the pack.  This means that the road to enabling UDMA has a few less twists than it did when enabling it with SP3. 

To enable UDMA in NT with service pack 4 you will still have to add the DMAcheck utility (or use third party drivers).  For some of us, (I include myself here) it will be necessary to not only install DMAcheck, but to force the registry as well.  There is a certain amount of risk involved here, but the majority of people I have spoken with have had no problems with forcing the value in the parameters registry key.  The problems usually are a result of enabling DMA on a channel with a non-DMA drive on it (such as an older CD-ROM).  A full "walk-through" of the installation procedure can be found in the original NT UDMA article.  The only difference is that when using SP4 is the extra steps of adding the idefix-i and atapi-fix patches are not necessary. 

Service pack 4 is a large file. At about 25 MB for the standard file (if you can find it in its entirety) and 31 MB for the version with 128 bit encryption, the download takes a while to get.  The 40 bit encryption version is available as a 6 MB download that querries your copy of NT for any uninstalled components and installs them for you.  Installation is straight forward and offered me no surprises when I installed it over SP3 with the patches and DMAcheck installed and the parameters already forced to 0X2; to enable DMA.  There was no need to change the registry settings again.

Performance
It had been a while since I had reinstalled NT and benchmarked the difference between no DMA and DMA enabled.  I have been using the Tyan 4021 drivers and have not benchtested my system (with NT) since I upgraded to the Celeron A.  I was again surprised at how poorly NT performs without DMA enabled.  The CPU utilization is extremely high and disk transfer rates are painfully slow without DMA.  The lack of native UDMA support with this OS continues to baffle me.

Testing
Windows NT was freshly installed to a FAT 16 formatted partition.  Aside from the benchmark programs, the only non-MS installed programs or drivers used were for the DCS sound card, the 4.50 Matrox G200 video card, Hayes modem, and the D-Link ethernet card.  All tests were performed 3 times and the results were averaged.  Between each test the partition was defragmented and the system was rebooted. System
Celeron 300A o/c'd 4.5X103
Abit BH6
Hitachi PC100 SDRAM (64 MB)
Matrox Millennium G200 Video (8 MB SGRAM)
DCS S805 A3D Sound Card
Quantum Fireball SE 3.2
Quantum Fireball ST 3.2
Matshita CR-574 CD ROM
D-Link DE-528 Ethernet Card
Hayes 5675 56K v.90 / Flex Modem
Inwin A500 Case w/ cooling modifications
WinNT 4 Workstation

Timed File Transfer
The last test was to test the speed in transferring files between the two hard drives in a real-world application.  For this test, I transferred a 105 MB copy of Win95 cabs and other assorted files between two 1000 MB partitions.  The folder was moved from drive 0 to drive 1 and back again.  This process was repeated three times for each configuration with the results being averaged.