Remove 3-way switch from a 4-way circuit?
To eliminate a 3-way switch, simply connect the feed wire to either one of the traveler wires, using a wirenut or similar approved connection device. Isolate the other traveler wire by capping it with a wirenut. You can identify the feed wire as it will connect to one side of the switch and the traveler wires will be on the other side. If you are not sure, in this case, it is safe to experiment: interconnect any two of the non-ground wires and isolate the other. If the circuit doesn't work, switch the isolated wire with either one of the others.
You noted that you will be replacing the switch with a switch for another circuit. One thing to watch out for is box fill. The new wires may possibly exceed the allowed fill. I'll leave it to a pro to expand on this.
Also, you may be aware of this already, but make sure to turn off the breaker for the circuit and test the wires for voltage before starting.