How can I safely connect a 240v shallow well pump to a switch?
I'd like to use a common double-pole switch (four-way)

Those are completely different things.  You just said &quot;Toyota Prius pickup truck&quot;.
A common 4-way switch is a &quot;reversing switch&quot;, 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 &quot;heavy duty&quot; 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 &quot;SWD&quot; or &quot;Switching Duty&quot;, 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.