Engine fully assembled. Cam sprockets does not line up due to thicker head gasket. checked timing with a timing light at TDC (white mark on pulley), advanced the timing +4, cam sprockets lines up. does this mean that the intake cam was off by 2 degrees? what is a logical explanation?

 

I don't quite understand what you're saying... You don't use a timing light to check CAM timing. You do that by lining up all the marks with the engine stopped.

After you fix the cam timing first, then look at spark timing. That's what you use the timing light for.

When you just say "timing" we don't know whether you mean cam timing or spark timing.