Using Lightroom 4.3 on a Mac Pro (OS X 10.8.3, 16 Gig RAM, NVIDIA GeForce 8800 GT 512 MB graphics card), I've noticed that when I use the adjustment brush on imported PSD files, there's an obvious tiling effect showing as I brush. The "tile" is a rectangle that's almost transparent, but has a bit of color to it; the tile moves with the brush. The effect is not permanent and does not affect the final image, only showing as I use the brush. It's an annoyance, but I wonder what's causing this.
This only occurs with imported PSD files, not the DNG files that I have in my Lightroom catalog.
It could be a bug or your graphics card driver.
Try Lr 4.4
Just downloaded 4.4 and installed it. Looks as though it solved the problem.
Thanks for reporting back.