Yesterday, my Airease furnace wouldn't stay lit so I cleaned my flame sensor and also my igniter... well I ended up breaking my igniter so I bought one and replaced it but now my furnace comes on for about 20 minutes and shuts off. The light then blinks 4 times. In the book it says that the fault is an "open limit switch or limit switch circuit" what is this? And how do I fix it?

There should be a wiring diagram located on the inside of the blower door. It will list all the components and show the wiring for your system. Usually a open limit switch or limit switch circuit problem is caused by

The control is defective OR out of range
The air filter for the furnace is dirty
The supply OR return registers have been closed or blocked allowing less air to flow in the system causing over heating.

If your system has not had Professional ON SITE service now might be a good time to have it done. Winter is getting closer and you would want your system to be in top condition.