Cornerpin adjustment

Hello,

I'm new to Expressions. And I think the question I have is simple. I have tracked the corners of a tv-screen with 4 null-objects (Mocha coud not the job). On my layer, witch has to fit the screen, a placed a cornerpin. The cornerpins are connected to the 4 Null objects using a script (pionted to the postition of the null). It works great, but I see a little screen edge. So I wanted the corner pins have a little adjustment in X an Y (4 pixels orso). So the layer, covering the screen , is a little bigger. But I can not find the right scriptcode. I added "

thisComp.layer("lb").transform.position9+(-4,-4)". But that did not the trik. Does anyone have an idea?

Thanks