An electronic firm finds that 4% of is defective.

this is not asking a question and i am not sure what the question you are asking is.

out of every 100 only 96 are useful

96 can be delivered out of 100

5000 can be delivered out of (100x5000) / 96 = 5208.33

as whole number next to it is 5209

Now to verify

4% of 5209 = 4x5209/100 = 208.36 will be defective

or 209 rounded off

5209 - 209 = 5000 ready to be delivered

So answer is 5209