Heres my 5c worth...
> On my PC I have 4 COM ports, and I can set the IO card to what I want, but
> 95 insist that COM3 stay on IRQ4, and COM4 on IRQ5. (Useless info : When I
> stick a modem on COM4 it talks to it, but the modem response isn't seen.) If
> however, I stick it on COM3, the same IRQ as COM1, my mouse, using the modem
> makes the mouse hang up. If I stick PIC comms there, the mouse hangs up, but
> the PIC talk isn't received unless you move the mouse !
I assume from the behaviour above that you do not have all 4 comm
ports on 1 card.
If they are on different cards, then each port must be on a
different interrupt, so as not to interfere with each other (see
previous mailings on subject)
You can change the Interrupt settings for COM3 and COM4 (and COM1&2
for that matter) from....
Control Panel -> System -> Device Manager -> Ports ->  COMx ->
Properties -> Resources.   (Phew)

I have a Tiawanese miltiport card that works fine, with all ports on
the card sharing the same interrupt (5).
One thing that puzzles me is that all ports have the "Settings Based
On" scroll box set to "Basic Configuration 8", which is different to
all other serial port settings.  Could it be that this is how Win95
works out how to share the interrupt with multiple devices.

HTH
Regards
John