Distributions from a Roth 401(k) are always a proportionate mix of contribution basis and earnings. You are not permitted to obtain a distribution that consists entirely of contributions if, due to investment gains, the balance in the account is greater than the amount of contributions in the account. The Roth IRA ordering rules do not apply to a Roth 401(k).
Using the distribution to pay taxes has no effect on the tax consequences of having received the distribution.
