The salary tax threshold is 1600 yuan, and the excess is taxed according to the following proportion:

Below 500 yuan: 5%

500-2000 yuan: 10%

2,000-4,000 yuan: 15%

Because the monthly salary is 4,000 yuan, the portion exceeding the threshold of 1.600 yuan is 2,400 yuan, so it should be taxed at 15%.

Then the tax collected is: 2400× 15% = 360 yuan.

Therefore, the monthly salary of 4,000 yuan after tax should be: 4,000-360 = 3,640 yuan.