I'm not 100% sure about the IRQ, but depending on operating system support, devices should be able to share an IRQ without any problems (when the IRQ is triggered, the drivers have to know it is shared and remember to check which device is responsible for the interrupt.)
The problem is that IDE also has I/O port addresses. These cannot be shared at all, so if you put two IDE controllers in with the same I/O address then neither one will work. This is exactly the same as serial ports - they can share IRQs, but it's the I/O address that defines which COM port it is.
For IDE, the primary controller uses I/O addresses 0x1F0 to 0x1F7, while the secondary IDE controller uses 0x170 to 0x177.
There seems to be some sort of consensus that a third IDE controller can sit at 0x1E8 - 0x1EF, and a fourth at 0x168 - 0x16F, but this is a much later development so there is limited (if any) BIOS support. No BIOS support means you won't be able to boot off any drive on those controllers, unless of course the IDE controller card has a BIOS that is aware of the third and fourth controller addresses. I believe XTIDE is, or at least can have the I/O addresses it responds to changed.
The main difficulty is of course finding an IDE controller that can have its I/O addresses changed. Most of the ones I have only have a single IDE connector and it can be either set to primary or disabled completely - you can't even set it to be a secondary controller, let alone tertiary or quaternary. Possibly controllers designed for RAID setups might have jumpers to change their addresses, but that's what you will be looking for if you want more than two IDE controllers in the same system.
The other possibility is that your motherboard could let you move the onboard controllers to positions 3 and 4, so add-in cards can provide controllers 1 and 2, but I think this is rare. You might think a server board would support this, but usually people who wanted to do this would switch to SCSI where you can put 7 or 15 devices on a single controller depending on SCSI version, so even server boards probably only support two IDE controllers.
As for using ISA cards, remember that a 16-bit ISA bus running at 8MHz can theoretically transfer a maximum of 16MB/sec across *all* cards. Usually it will be significantly less than this. A single cheap SATA SSD from China with an IDE converter, with an onboard IDE controller capable of UDMA2 will max out at 33MB/sec - double the speed of anything connected to ISA, with no RAID involved, no noise, and a fraction of the power draw. So fair enough if you are doing it just to see whether it can be done, but if you want any sort of decent performance for what the machine is capable of, an ISA RAID array is definitely not the way to go!