The reason the modem installs on COM4, even though you don't appear to have one, is because the internal modem has, in it's simplistic terms, it's own COM port built in, hence COM4.
COM4 shares the same Interrupt as COM2, and providing nothing is using COM2 it shouldn't cause any problems (although in practise you can use COM2 and 4 at the same time, but can have problems). However on many newer PCs, you may find COM4 is using a different IRQ to COM2.
If the modem is working OK, and I would leave it at COM4, rather than mess about trying to get around it.
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