No, you don't have to report 401K loan. Funds
borrowed from your 401K plan are not treated as taxable, as long as you are
paying back the loan. If you default on the loan, then it will be taxable
distribution, which you will have to report.
However, if you took a distribution (not a loan), it must be reported. You would receive form 1099-R for it and must report it as such on your tax return.