I got it , you are almost correct Bhagyashree Mukherje
Before the 2nd transmission(i.e after the 1st RTT the window size is becoming 4kb )
Before the 3rd transmission(i.e after the 2nd RTT the window size is becoming 8kb )
Before the 4th transmission(i.e after the 3rd RTT the window size is becoming 16kb )
Before the 5th transmission(i.e after the 4th RTT the window size is becoming 24kb )
So only 4 RTT needed to reach 24kb window size.