4 hours is 4*60=<<4*60=240>>240 minutes.
If the doubling time is 20 minutes, then after 240 minutes, the number of bacteria would have doubled 240/20=<<240/20=12>>12 times.
Thus, starting with only 1 bacterial cell, after 4 hours, there will be 1*2*2*2*2*2*2*2*2*2*2*2*2= 4,096 bacterial cells
#### 4,096