I'm hoping @pk or @dmertz can help follow up here, in case I make a mistake.
For a Roth 401k, that is taxable according to US law, because you were working in the US when you made the contributions. That means that non-qualified withdrawals would be taxed according to US law, even if you are living overseas. Withdrawal of contributions before age 59-1/2 is not taxed, but withdrawal of the earnings is income tax plus 10%.
For a pre-tax 401k, you can probably leave the money in the 401k, you don't have to roll it over. Most of the time, the company won't just kick you out. However, you certainly have the option of rolling it over to an IRA. Doing a Roth conversion now could save you taxes later, but this depends on your state of residence, and your home country's tax rules.
For the IRS (federal government), a Roth conversion will likely be taxed at 12%, 22% or 24%, depending on your other income, as long as you are still an RA. If you withdraw the money later as an NRA, it is taxed at a fixed rate of 30%. So just thinking about the federal government, you would pay less tax to the IRS by converting the money while you were an RA. However, we also have to think about state income tax. As an RA, you will pay state income tax on the conversion for the state you are living in, if you withdraw the money after leaving the US, the state doesn't tax it. So if you are in California, for example, and paying 10%, you may pay less overall tax by withdrawing after you leave the country and paying the flat 30%.
And you also have to consider your home country. If your home country taxes Roth IRA withdrawals, then you pay taxes twice, once to the US when converting and once to your home country when withdrawing, and because you don't owe US tax on the withdrawal, you can't file for an offsetting US tax credit. If you withdraw regular IRA funds, you also pay taxes twice, to the US and your home country, but the US will usually give you a tax credit to (at least partially) offset the foreign taxes you pay on the same money.
Also remember that conversions start a new 5 year clock plus the age 59-1/2 rule. Even if you convert your pre-tax 401k to a Roth IRA and pay the taxes today, you will pay income tax on any new earnings you withdraw before age 59-1/2, plus a 10% penalty on the entire withdrawal if withdrawn in less than 5 years.
So I think you need to find out if China will tax your Roth IRA withdrawals, and you need to think about your state income taxes for your current US state of residence. If China will tax the Roth IRA withdrawal, I think a conversion will probably hurt you in the long run, even though it may save on US taxes.
Lastly, let's think about how the foreign tax credit works. You won't ever pay less than the US tax. For example, suppose China taxes IRA withdrawals at 20% and the US charges 30% to a NRA. The US would give you a 20% offset so you pay 10% to the US and 20% to China, so the net result is the same 30%. And the 10% penalty for early withdrawal is a penalty, not a tax, so it is not offset by foreign taxes. For this reason, if you are planning on withdrawing the money before age 59-1/2, I suspect that it makes the most sense to do that before you leave the US and pay the taxes once and get it over with. If you will leave the money until after age 59-1/2, then you have to next find out if China will tax Roth IRA withdrawals.
