the requirement to become a US citizen is to stay half of the 5 year period immideately before the date of application in the US. If you stayed the first year in the US, and then spent another 4 years in China, you are missing 1.5 years. Therefore, in the situation above, the best time to come back is after you stayed 2.5 years overseas. That way, you can take whole advantage of the first year you spent (or wasted) in the US.
