Finally, after many failures, I have my system (Compaq Dekpro 4000, P233MMX, 128MB RAM, 2 * HDD) quad-booting DOS, Win98, WinNT 4.0, and OS/2 3.0. Each OS is on a primary partition, and there is also a primary partition for the Compaq setup software (thank, Compaq!). The partition layout looks like this:

HDD0:
8MB primary - Compaq Setup
2000MB primary - Win NT 4.0
2000MB primary - Win98
unpartitioned space (will become extended)

HDD1:
2000MB primary - OS/2 3.0
2000MB primary - PC DOS 2000
unpartitioned space

I had a few challenges setting this up, but the biggest one was that the setup routines for NT, DOS, and Win98 only want to allow installation to a partition on HDD0 AND that I wanted no more than 3 primaries on each HDD to allow room for me to create and extended partition for data drives. Of all the OSs, OS/2 has the most flexible FDISK, so I've used that for all partition management. I was using Partition Magic, but ran in to some issues with NT not wanting to install in to a partition created by PM - I haven't investigated this deeply. I'm using AirBoot 1.06 as a boot manager.

I tried quite a few different installation/partition orders, but ended up with this one because DOS is dumb. That ended up being the key, since even though DOS Setup would only allow installation on the first Primary DOS partition on HDD0, once it was fully installed I could format, sys, and copy DOS to the other three partitions and have them all remain bootable.

From that point on, it was reasonable straighforward:

1. configure AirBoot to hide all primary partitions other than the target from each OS
2. install each OS to it's target partition (which it enumerates as C: due to step 1)
3. repeat until done

It took a few tries because OS installation tends to mess with the MBR and/or partition table, so it was necessary to reinstall/check/fix AitBoot fairly often. I eventually just got in the habit of running through the options, especially partition hiding, at each reboot to ensure that only one primary at a time was visible. Not doing this set me back to the start at least a few time.

The last wrinkle was that different OS installation routines display partitions in different order - by index, in order on disk, etc. That led me to install an OS to the wrong partition more than once. That's why I eventually started by installing DOS to all four primaries - that way, if I made a mistake, I'd still end up with one bootable copy!

Next up, installing/configuring drivers and patches for each OS.

In case anyone isn't familiar with AirBoot: https://sourceforge.net/projects/air-boot