Is it OK to borrow from my 401k to give to an investor who will supposedly set me up with a high rate of return, and then pay back my 401K? What will I need to look for before doing so?

Depends on the specific rules for borrowing from your 401(k) per your employer's plan. Be aware that most plans require you to pay the loan back in 5 years or less, and if for some reason you can't pay it back the loan is connsidered a distribution, which means you will suddenly owe income tax on it plus the 10% early withdrawal penalty. Also the money you pay the loan back with is after-tax dollars, as opposed to pre-tax which the original contributions were made with, and when you retire and pull that money out you will have to pay taxes on it again. So work that into your payback analysis and see if the scheme makes sense. In gneral I would say that if this investment idea is so good, come up with cash from other savings rather than your 401(k).

Good common sense, Your 401K first is an investment.

So just how much profit are they telling you they will pay.
Sorry it sounds more like someone is trying to scam you with one of those 12 to 20 percent profit

A good 401K should still be making that 5 to 6 percent ( some even more, I made about 12 percent profit with mine last year)