That's a good question that I'm not sure if know the answer to.
By nontaxable conversions, people generally mean conversions from IRAs where the money in the original IRA was nondeductible.
There remains the question of how do Roth 401k rollovers count for the purpose of withdrawal order. Is it order number 1 or 3 in the withdrawal order.
According to this site: http://www.investopedia.com/articles/retirement/09/roth-401k-rollover.asp, Roth 401k rollovers are subject to the 5 year holding period that satisfied if a Roth IRA has been open for 5 years (holding period of Roth 401k does not matter).
This site: http://www.fairmark.com/rothira/distrib.htm
indicates that rollovers of nondeductible 401ks (different from roth 401ks) are treated like conversions, and might indicate the same of Roth 401ks, but it is unclear.
If I had to guess, I would say like number 3, but if the answer is important, I suggest asking someone on the Fairmark forum.
I'd like to add to/correct my previous post to say that the withdrawals from conversions are done on a FIFO basis(first in first out - taxable from conversion 1, nontaxable from conversion 1, then taxable from conversion 2, nontaxable from conversion 2...)