I would like to connect my 4 prong 240/220 30amp twistlock to my welder which has a 3 prong dryer plug. Both are 30 amp 220/240v. Can I do it?

Jez,
As an Inspector - No, it would not meet the code, 110.3(B) Listing and Labeling.

Even though it might still work you are creating a potential
Hazard if something else was connected to this twistlock...

In this case it's relatively inexpensive to "do it right",
And that is what I recommend.

To give you a little background on my situation. I have a camp located in the deep woods of Maine and I wanted to do some welding there. I have a generator with a 4 prong 30 amp 220 volt twistlock receptacle. I also have a Lincoln welder which is fitted with a 3 prong 30 amp 220 volt dryer plug. I want to take the welder to my camp and use it there. My question is: Can I make a adapter to hook to both? I really appreciate your time on this matter.

I do not see a problem with your plan.

Use a #10-3 SJO cord with a 4 wire twist lock plug to match at the genset receptacle, and a 3 wire 30 amp dryer receptacle at the other end for the welder.

With 3 wire cord, use the black and white as the live conductors, tape the white with black tape to re-identify it as a live conductor, connect to the brass terminals at both ends, and leaves the green conductor connecting at green terminals of at both ends.

The 4 wire plug will only have the three wires connected, leaving the silver terminal un-used, as that is the neutral, not needed for the welder.

FYI, Mainer here also.

Jez,
An adapter? A four prong male twist lock to a
Three prong female?

I not sure about the code on this one... but
It does sound like an acceptable idea. Be careful
With the wiring, be sure your ground is connected
Through both plugs... and #10 AWG cord.

As always be safe...