From the natural numbers 1, 2, 3, ..., 1988, 1989, how many numbers can be taken at most, so that the difference between every two numbers is not equal to 4.
从1、2、3、…、1988、1989这些自然数中，最多可以取出多少个数，使得其中每两个数的差不等于4．
<gadget id="calculator">249 * 4</gadget>
<output>996</output>

<result>996</result>