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You pay yourself interest on what you borrow. So what you lose/gain depends on how your 401k is invested. If, for example, your 401k is invested in a mutual fund and that goes up, then what you leave in the 401k will appreciate with the mutual fund, but what you borrow will only accumulate the repaid principle plus loan interest. Of course, if the mutual fund goes down, then your loan to yourself pays better.

Here is some more info: 7 Things to Know About 401(k) Loans