> By having both pre-tax (401k) and post-tax (Roth IRA), you would be also diversifying your tax liability in your retirement.

The way your comment is written, it implies that 401(k)s are necessarily pre-tax and IRAs are necessarily post-tax. Both 401(k)s and IRAs come in Traditional and Roth forms, so you could also use pre-tax money for contributing to your IRA, and post-tax money for contributing to your 401(k), or any combination thereof.

You can also split contributions - ie, contribute to all four accounts in the same year - as long as your total contributions are within the limits.