Puzzle ZOIC 

Workings

Hint
How many minutes are between the times?

Answer
50 minutes.

Reasoning
Between four o'clock and eight o'clock, there are 240 minutes.

In these 240 minutes, there are 4 lots of the unknown time (t) plus 40 minutes.

Therefore, 240 = 4 x t + 40.

Therefore, t = 50 minutes.