I just wanted to let all of you know that there is an fast and very inexpensive fix to your 4 button stat that has issues with the buttons not working. My fan button quit working and as you know that witout it you cannot toggle between zones. Upgrading to the 5 button system is too expensive for me right now. I located some people on ebay that had reconditioned thermostats that ranged from $155.00 down to $60.00. The guy that had them for $60 was a bit reluctant but would sell me a single switch for $15.00. I figured if he was able to get switches then so could I. I found a post on irv2 that listed the manufacturer part numbers. I found Mouser Electronics in Texas that sold the pushbutton switches. I ordered 4 of them for $1.22 each. It cost just under $10 for the 4 switches and priority shipping. The switches arrived yesterday. You'll need a small jewelers screwdriver to do this. Being careful the top of the switch comes apart by gently lifting the plastic tabs on the side of the switch. Be sure to observe the tabs and what side they are on. One tab is a bit longer than the other and it WILL make a difference when it time to reassemble. There is also a small spring in there that can be lost easily. When its time to put the new spring in if you look carefully at the base and the top section you'll see where the spring is placed. The chrome contact strips will come right out of the base. just insert the new contacts, spring, and snap the top back on remembering the the original position of the tabs. It sounds like a lot but it only took me about 5 min to replace the guts of the switch, reassemble and test the strat. The system works like new.
Private message me if you want more help with this.