Thanks for the extra information, that is very helpful.
Even though your BIOS sees all 4 drives, and you had used a CD drive, I would try swapping in a different IDE cable. Sometimes just that last plug and un-plug operation is all it takes to damage an IDE cable.
Also, try a different power connector.
Just as a reality check, this drive has been partitioned and formatted as a FAT32 volume?
What is on the second partition of the Primary Master Drive?
By default, Windows is going to attempt to assign the new drive as F and re-assign the second partition on the Primary Master drive as G
Here is a good question, did you remove the CD drive from device manager before you shut down?
If not, go to the ENUM key in the registry and remove the entries for your CD-ROM drive.
That might be all it takes to fix the problem.
Or, if you don't want to edit the registry, swap the CD drive back in, and change the assigned drive letter to something high like R.
Reboot twice.
Open device manager and remove the CD drive.
Shut down and swap the hard drive back in.
I run up to 4 hard drives and 3 optical drives on my XP system (extra IDE controller on the motherboard) with no problems.
2 additional thoughts:
Were ANY of the drives set up using the drive manufactures utilities?
This can cause strange problems when all drives are not from the same manufacture.
While I am not aware of any restrictions on the number of physical hard drives on W98, I wonder if 400GB exceeds some addressing or file structure issue for W98.
Similar to the large RAM bug.
Speaking of which, if you have over 384MB of system RAM, drop back to 256MB or 384MB and see if you are bumping into Windows needing the upper (above 512MB) memory addresses to deal with your total drive capacity.
I don't have a system handy that I can play with 4 large drives on, or I would see what I can find.
Chas