Pete Marshall's fix worked for me. I have three monitors - L, M, R. I'm using Lucid Logix on an Intel 4000 with a a dedicated graphics card. My two main monitors are connected to the motherboard (HDMI and DVI) and the third comes off the HDMI on my graphics card to my TV. LR4 was always putting the full screen secondary on the the TV (R) rather than my second monitor (M). When I put the main program on M, the secondary monitor became L. The analysis that the secondary is always to the "left" (wrapped) of the main window within the set of monitors seems to be accurate.
Fortunately both of my monitors are identical and perfectly calibrated, so doesn't matter to me which is which, as long as its not on the TV! 