How do I make an extension cord from my 4 prong 240 twist generator outlet to a 3 prong 240 outlet for my radial arm saw?

You need a 4 prong plug to match the generator, then just hook up the 2 hots and the ground, forget about the neutral, then a receptacle to match saw.
A breaker between would be good if the Generator had 50 AMP receptacle and saw was 20 amps? How many amps is the receptacle and how much does the saw require?

You'll have to make a conversion cord with the correct ends for each. Just do not use the neutral terminal at the twistlock plug. You'll need to use just the two hots and ground for your straight 240v saw.

Wow. That's spooky. :)

What's spooky?

Almost the same post at the same time.

I was afraid I said something Scary.