Your question is confusing.
You can buy and sell investments within the 401k plan, and there are no tax consequences until you withdraw money (usually after you retire). The kinds of investments you are allowed to invest in depends on the rules of your 401k. If you find that t-bills are allowable, you can allocate your money that way instead of stocks or mutual funds.
However, if you take money out of the 401k, it is immediately taxable (plus a penalty if under age 59-1/2) no matter what you plan to do with the money after.
