4000, think about it. If the tens place rounds the hundreds up, you add one hundred and drop the tens, ones, etc. It's the same thing with the digit 9, only the next number is 10, so you have 3994+100, which equals 4094. then you drop the tens and ones to get 4000.
853 rounded to the nearest 100: 900. 10853 rounded to the nearest 100: 10900.
100
600
100
100
0
555 rounded to the nearest 100 is 600.
97 rounded to the nearest hundred is 100. 97 rounded to the nearest ten is 100
100
100
100
100