(2) The frequency distribution histogram is as follows:

(3) The proportion of students with [60,90], that is, the frequency of students with [60,90] is 0.2+0.3+0.24 = 0.74 = 74%;

(4) The proportion of students with grades below 85 is the frequency of students with grades below 85. Let the corresponding frequency be b, then b=0.72.

It is estimated that 72% of the students' grades are below 85.