Epson 4000 - Manual (Page 102)

Note
Not all hard disk drives can take advantage of this feature.
To take advantage of the local bus IDE interface, your hard
disk drive must support a 32-bit data path that utilizes
double-word I/O. To find out whether your hard drive
utilizes double-word I/O, see the hardware specifications
for the drive or contact the vendor of the drive and request a
product specification.
6-6 Installing VGA and IDE Drivers