Well then. You've ben had. 4,000kb/sec isn't even close to 16x. Not to mention the fact that you're not even getting that speed anymore.
Maybe your drive is just crapping out on you.
Either way, here's how to enable DMA:
Right click on my computer, Properties, Hardware Tab, Device Manager, find the IDE ATA/ATAPI controllers section, right click on the channel, properties, advanced settings tab, and select "DMA if available".
Oh yeah, and it's NEVER good to have a CD or DVD-rom drive on the same channel as an IDE drive. If your CD/DVD-ROM is UDMA-33 (which pretty much all of them are) then your hard drive will be running at that terribly slow speed as well.