it depends on your plan -- review your plan documents and ask your plan administrator. 

With mine, I cannot withdraw unless 1) there's a hardship (which is really not the case here; and 2) I take a loan from it first.  Even then, there would be a tax penalty.  I can (and may) take a loan from it, but the catch is that if I were to change jobs, I might have to pay the loan back in full.  Very risky. 

Yours may be different.  Some Roth 401ks, for example, will allow  you to withdraw an amount for a house downpayment without penalty.  But again, you need to check your plan documentation.