I'm using Lightroom 4 RC2
I have one monitor displaying the modules and one monitor in loupe view. I can make changes to an image on the develop panel and see the changes happen in the image, but on the other monitor in loupe view, nothing happens.
My Lightroom 4 experience has been horrible. It is much slower than LR3 and I'm really disappointed in Adobe and myself for wasting the money on it. I would've rather just paid the same amount and had LR3 support the 5D3 and D800.
Short answer: there's a bug in 4.1RC2 when using the second monitor if the Standard Preview size (Catalog Settings) is too small. Increase the preview size and the problem should go away. Note also (at least on Windows, can't say about Mac) the bug is accompanied by a severe memory leak which will also negatively impact performance.
Read my post again....I didn't write about what size preeviews you had generated, I said check the Standard Preview size that you have in your Catalog Settings (see screenshot). If you are able to, try increasing the setting to one or two levels higher and see if that makes a difference. In my case my setting was at 1024, increasing to 1440 did the trick. Others have had to increase from 1440 to 1680 or 2048.
Thanks, that looks to have helped it for now. Now I'm noticing that the colors are very different on the loupe view. The images are much cooler with more blue in them. I can switch which monitor the loupe view is on and the problem persists, so it's not my monitors.