The info from Myke Predko seems to be the most accurate I've seen so far.
Here is some practical info.

------------------------------------------------------
Typically (but not necessarily)
Comm1 and Comm3 ---> Int 4
Comm2 and Comm4 ---> Int 3
------------------------------------------------------
but...
You can set it up differently if you want to.

You set it usually in the CMOS/BIOS setup screen on your PC's or via
jumpers/DIPS on a Comm board.

Now Comm1 and Comm3 "share" an interrupt but "share" is really being
optimistic. For most purposes you really can only use (Comm1 OR Comm3) and
(Comm2 OR Comm4).

Next, you can change interrupts in Windows control panel but it only tells
Windows what interrupt to expect. The hardware will continue to use what it
was set to.

Windoze 95 takes this away but it didn't set the interrupt anyway so we
didn't lose much.

There is a problem with PC's and the allocation of interrupts( there aren't
really enough). They designed the PC so that every stupid device needs its
own interrupt. If you have a Sound card, Network Card and your 2 comm ports
are full you are pretty much done adding hardware (unless you want to get
funky). PS/2 mice help as they use a higher interrupt that is usually free.

To add more serial devices you can use a multiport serial card. These allow
multiple comm ports to use one IRQ ( the hardware in the card makes this
possible).

 Bottom line....PC's kinda suck. I don't think IBM was thinking ahead to
Pentium/sound blasters/networks/64MB mem/trackballs/hand scanners/etc...
when they designed the PC. There is help on the way with USB and Firewire
(I hope these work better than Plug and Pray).