Three-quarters of oil from the initially full 4000-liter tank would be 4000*(3/4)= <<4000*3/4=3000>>3000 liters
In addition to the 3000 liters that were already in the large tanker, there are 3000+3000 = <<3000+3000=6000>>6000 liters in it now.
The tanker would be filled to half capacity by (1/2)*20000 = <<(1/2)*20000=10000>>10000 liters of oil
There are 10000-6000 = <<10000-6000=4000>>4000 liters left to reach half capacity.
#### 4000