Since my Roth 401k contributions are made after-tax, can I withdraw from my Roth 401k account at any time and without taxes?

No. The restrictions that apply to a Roth 401k are the same as those that apply to a 401k. You may only begin to withdraw from your Roth 401k account without penalty under these circumstances:

• You've had a Roth 401k account for 5 or more taxable years.
• You are 59 1/2 or older.
• The Roth 401k account holder is dead.
  
• You are disabled.

If you meet these criteria, you may start withdrawing from the Roth 401k account without penalty and without taxes.