I have a generac generator with a 4 pole twist-lock 230v receptacle. I need to plug in my 230v 3 pole arc welder. Can I just clip the white wire?

3 pole welder? If the welder needs "3 pole" it is 3 phase, and I suspect your generator is single phase, and the welder will not operate.

If both are single phase, then you can cap the white neutral.