I THINK the answer is "whatever pulls the most CFM's." The size of your building (L, W, H) as well as where in that area the welding is taking place within that space all come into play. BUT, you said 4" outlet, so...something like this comes to mind:
https://www.amazon.com/VIVOSUN-Inli...&hvtargid=pla-314983957332&psc=1

A friend of mine uses a bathroom fart fan in a 18' X 24' shop with an 8' ceiling, placed over in the far corner, and it does OK, but I'd go with something better.