The bacteria population doubles in 4 hours.
So, in 1 hour it increases by x/4 of its population, where x is population.
So in 1/2 hours or 30 minutes, the increase will be x/4 × 1/2 = x/8
=> if we start with 4000 bacterias, population increase after 30 minutes is 4000× 1/8 = 500
So, 500 more bacterias were added
Total bacterias = 4000+500 = 4500 bacterias