The average score of the first three times is 89, so the weight of 89 is 3.

Suppose the math score in the fourth exam is X. Compared with the previous three exams, the weight of this exam is 1.

So, the average score of the four math exams is:

(89*3+x* 1)/4=90

Solution: x=93

The math score in the fourth exam is 93.