3 prong plug to 4 prong outlet adapter? (Welder outlet)
The simple solution (unless you drag it to other places with different receptacles) is to put the appropriate 4-prong plug on the cord, with the neutral prong of the plug not connected.
Another option is to add a box next to your box with the 4-prong receptacle and install the correct 3 prong receptacle in it. If you plug in and operate two things at once you may trip the breaker, but it spares you the &quot;extra cord and plug&quot; that your self-made adapter would have.
Edit to add: If you don't actually have something that uses the 4-prong receptacle, you could just replace the receptacle (capping the neutral wire in the box) and (if you are nice) write a note on the wall that there is a neutral available if someone wants to change the receptacle back to 4-prong later.