It takes light from the closest star to earth (other than the sun) about four years to reach earth. if intelligent life were on a planet circling that star, how long would it take for scientists on earth to send them a radio transmission and for the scientists to receive their reply?

If the intelligent aliens on that distant planet could receive our signal, and then reply to us INSTANTLY with no delay, then our Scientists on Earth would hear the response eight years after they sent their original greeting.

Light and radio waves both travel through space at the same speed.  If our scientists sent a radio transmission to that distant planet, it would take four years for the radio message to get there and be heard.  Any response would take four years to travel back to us.  If the aliens didn't dilly-dally and sent their response immediately, the round trip would total up to eight years.