It is over 3 years so Apple probably will not change the LCD. (you might try but it is not probable)
You should take a picture how the problem looks like and post it here. We can take a look.
1. You should remove the front panel with sucking glass-lifting tool. You can find video's on YT how to take out the LCD, you have to disconnect 4 kabels. You can clean the LCD unit and clean the whole machine inside (with some air can or compressor or just blowing it by yourself, compressor is the best).
After 4 years the mac surely very dusty inside it is anyway good to clean it.
2. If it does not help you can try to take apart the LCD unit and clean it inside it is a very delicate procedure but it can be done look video on YT also. I did it myself two times.
3. If it still no help that it might be that something is overheating inside the iMac, this was my case with my dying HD, I replaced it and whoala the marks I had disappeared. But your case might be different. I did not found too much dust inside my LCD others had more, dustier environnement or smoking whatever...