how to track an object that only appears at the middle of a footage (traveling in)

Hello People!

I´m having a problem here.
I have a simple footage that is only a traveling left to wright and at only in the middle of the clip is when a monitor appears.
My problem is to track the monitor because I want to replace the monitor (black) with a video (well I have the monitor turned off with four tracking points, one in each corner.
I dont now how to track something that is not allways in the footage.
Can you help me?

Thank you in advance

RP