You can roll a 401k plan over into a Roth IRA. However, when you
do so, you will have to pay ordinary income tax on the amount
rolled into the Roth.
Even so, a Roth IRA will usually perform better over time, as
the money not only grows tax free, but is taken out tax free as
well.
There are some great calculators out there that will show you
the impact of conducting this rollover. See attached link.