Theoretically yes, in practicality no.
Most 401k plans include a money market fund, basically a savings account with variable interest, none of which have broken-the-buck (yet). Also, a number of plans offer Guaranteed Investment Contracts which offer a guaranteed rate of return. Many of plans offer a selection of stock, bond, or hybrid mutual funds. Pretty much all of which have been getting hammered recently. There are short term investment grade bond funds (a very conservative investment) which are down ~4%. Gov't bonds haven't been spared, as a popular TIPS fund is also down ~3% YTD.