I'd like to use a common double-pole switch (four-way)
Those are completely different things. You just said "Toyota Prius pickup truck".
A common 4-way switch is a "reversing switch", it does not disconnect anything at all. It won't disconnect the motor; it won't even reverse a 1-phase motor, just cause a huge voltage spike if you tried lol.
A double-pole switch is exceedingly uncommon and I wouldn't expect a home store to stock one.
Regardless, you don't need a double-pole switch, a single-pole will suffice if it's rated for 240V. So you'll be up in the $10 "heavy duty" kinds.
to allow us to disable the pump when we're not on the property
I quite agree with Jeff, however. If your goal is an occasional disconnect, just flip the breaker off. I do that all the time with fridges and water heaters. Technically you should use a breaker rated "SWD" or "Switching Duty", but any new breaker should be (am I right ThreePhaseEel, does that apply to 2-pole breakers?)
Should I hard-wire the pump's cord into the switch in a junction box, or should I install an aftermarket 240v plug? If the latter, what plug type is appropriate?
GOOD question, many would misuse a 120V NEMA 5-15. Heck the entire nation of the Philippines does exactly that lol.
The correct plug-socket for a 240V-ground-not-neutral connection is the NEMA 6 family.
Doubling voltage halves current, so the correct plug is a NEMA 6-15. For sockets:
A 15A breaker requires 15A sockets (NEMA 6-15).
If the breaker is 20A, and there is only 1 socket (simplex), it must be NEMA 6-20.
If 20A and 2+ sockets, you can have any mix of 6-15 and 6-20 sockets.
Note how the 6-20 socket is keyed to accept a 6-15 plug. All these here are double (duplex) sockets, so one duplex 6-15 will work with both breaker sizes.