"Good standrd of living" has very different meanings for different people. Can you keep yourself reasonably well-supplied with food and drink and smokes? Yeah, probably, especially if you drink Chinese beer, smoke Chinese cigarettes, and eat Chinese food in small, cheap restuarants. The western brands for each are much more expensive, and in Gansu province likely hard to find as well.

If you are the kind of foreign teacher who seeks a regular diet of Pizza Hut, Budweiser, and Marlboros, 4000RMB will disappear rapidly -- especially if you begin ordering food after having several beers.

I assume the school is providing your housing and paying ALL utilities? If there is a "cap" on your utility allowance, be prepared to pay a significant portion out of your own earnings (in Gansu, especially in winter).

The real question -- at least to me is: Will you have anything left (finanically) when it's time go back home? I seriously doubt it.

My first job in China in 2003, was at a vocational college in a medium-sized city in Jiangsu province. At that time, the salary was 4500RMB per month; and in those days, I didn't drink beer and I have never smoked, and still the money went quickly.

Be prepared for possible mysterious deductions from your salary that might not be justified by more than some oral reasoning/explanation. Documented evidence (utility bills for example) is almost impossible to come by.

Keep as much cash in reserve (for the unexpected) as possible.