Furnace flashing 4 times (Ducane)
NEVER JUMP A LIMIT SWITCH TO ALLOW A GAS APPLIANCE TO RUN. THIS IS ONE OF THE MOST IMPORTANT SAFETY IN THE SYSTEM AND IS REQUIRED FOR PROPER AND SAFE OPERATION OF THE UNIT.

Limit switches may only be jumped by technicians to test and rule out certain components and are not to be left bypassed.

Your furnace seems to be going off on High limit. When the fan runs for long enough after the furnace shuts down on high limit, it will cool the limit switch to the point where it can close again and re run the heating cycle. This is why the unit is cycling.

There are many things that can cause this. Listed in order from most common to least common:


Blocked or dirty furnace filter (Most common)
Plugged heat exchanger (causes heat exchanger to over heat)
Blocked or dirty air conditioning coil 
Faulty high limit switch
Blower motor will not run (can be numerous causes)
Blower motor not spinning fast enough (Cause by dirt and dust getting into motor)
Faulty control board


Always check your furnace filter first. This may save you the cost of a service call. If your filter is clean then have a licensed HVAC service technician look at and repair the unit. This problem will either be a very very minor issue, or a very very major issue.