If the bacteria doubles its volume every 4 minutes and we know that at t = 57 minutes the container is full, we can work backwards from there:
If the container is full (V = 100%) at t = 57 min., then it has to be half full (V = 50%) 4 minutes earlier, as it doubles every 4 minutes.
Then, the container was half full at time t = 57 - 4 = 53 min.
Then, if we repeat this analysis, the container would have been filled to a quarter (V = 25%) 4 minutes earlier than it was half full.
That corresponds to time t = 53 - 4 = 49 min.
Answer: 49 minutes.