2% of the 2500 are defects. That means there are .02(2500)=50 defects in the batch.
The probability the batch is rejected is if AT LEAST ONE is a defect.
So, the easiest thing to do is use the complement and find the probability that none are defects and subtract from 1.
Therefore, we have to choose 4 from the 2450 that are good and 0 from the 50 that are bad. We choose 4 from 2500 altogether.