Four lights blinking on control board

[BigK75]

Hey Guys:

Question for you.  I live in a duplex and the unit below also has its own furnace that is now roughly 15 years old.  Anyhow I noticed that it stopped working and it would get a four flash error code on the control board.  I called the company that replaced my furnace upstairs and they came out and changed the pressure switch and cleaned the igniter.  This worked for a about a week or so and back to the same problem.  When the problem starts everything starts up correctly.  First the fan, then bright orange light (ignition?) and finally the gas which turns on and immediately turns off.  I called the company back to tell its not working again.  They came out switched the same part out (pressure switch) and told me that I either a) change the control board b) replace the furnace.  Anyway at this point after paying $360 bucks I have lost all faith in these guys.  I seriously don't think they care about fixing it.  They rather just replace it.  Since I just spent a wack of money on the furnace for the upstairs I am not eager to do it again.  This is a small until around 600-700 sq feet.  How would you handle this situation.  Would you get someone else to come and look at it or call them back and complain.  If I were to replace it can anyone ball park roughly how much this would cost. 

Claude
     

[walker]

They are fixing the part and not the problem.  They can throw parts at it all day, but if they cannot diagnose whats actually causing the failure then they will never fix it.  I would ask for refund on the services provided since they have yet to fix it.  They replaced the same pressure twice inside of a week and they are determined that the switch is the problem?  Get a real technician out there and tell them to keep their "parts changer" at home.

[Sergroum]

Yikes. That's pretty bad. Was it the same technician who did this, or was it an entirely different one? Did you tell him that someone changed the pressure switch already less then a week ago?
The control board could be a problem, but I would find it unlikely. Sometimes it does give a false positive on a pressure switch, but then it'd do the same with the brand spanking new pressure switch as well. 

The unit could be draining bad, or something is going on with the venting. There might be some intermittent loss of voltage on the wires to/from the pressure switches. The tubings could be plugged. The slope could be bad. I dont know. There's a lot of stuff that a normal tech would check and confirm before being all, "Newp. Change the furnace!"  Of course, if that furnace is 15-20 years old, then maybe in the end, it's still wiser to do it regardless.

[Frosty]

To get a better understanding,what is the make and model of the unit?