I have an old style honeywell [CT87N] It has three wire slots but my old one has four wires. It was ripped of the wall so I'm not sure what wire went where.There are light green,blue,white and red,the blue,red are connected at the furnace as well as the white and green. Where and what colors do I connect at the wall and will the new one even work with what I have. Thank You J.R.

"I have an old style honeywell [CT87N] It has three wire slots but my old one has four wires" a bit confused , did you mean to sat the NEW one has four wires? Read the 3rd post in this thread https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-faq-northerheat-58313.html

Should be pretty simple. Somebody just used two wires for one connection on both the r and w. Hook the red to r on the stat and furnace, and the white to w on the stat and furnace, leave the others unhooked. Should be good to go this way.

Hi
First check on the furnace which colour wire is connected to which label (alphabet)
And then match the same on the stat