At 4 o'clock sharp, the hour hand and the minute hand form an angle of 120 degrees. After that, every time the minute hand walks 1 minute, the angle of the minute hand passing is 360/60=6 degrees, the angle of the hour hand passing is 30/60=0.5 degrees, and the hour hand and the minute hand form an angle of 90 degrees for the first time.

(120-90)/(6-0.5) is about 5 minutes, that is, 4: 05;

When the hour hand and the minute hand form a 90-degree angle for the second time,

(120+90)/(6-0.5) is about 38 minutes, which is 4: 38;

4:38-4:05=33 (minutes)