Develop uses linear ProPhoto (ProPhoto primaries, but gamma 1.0). Library uses Adobe RGB jpegs.
If you can see any difference between them, that's normally an indication of a problem with the monitor profile.
A wide gamut monitor does extend very slightly beyond Adobe RGB in some colors, and so could be able to show a small difference in gamut between those two. But it would be more theoretical than practical. I'd go after the monitor profile as prime suspect.
With a standard gamut monitor, there's no way you should see any difference.
Thanks for your Answer twenty_one. The Monitor (ASUS P246q) is a wide gamut display and I use the ICC profile from ASUS. I see only a difference on pictures with extended colors. So, if I understand you right, that's normal.
How does it work in Lightroom 3? There, I never saw a difference between Develop and Library.
The Monitor (ASUS P246q) is a wide gamut display and I use the ICC profile from ASUS
Manufacturer-supplied monitor profiles are notoriously inaccurate and outright defective. They simply cannot be trusted. Do yourself a favor and buy a good calibrator, and this problem should go away. Yes, it's an added expense, but once you've made the (small) investment you'll never look back.
Good candidates are x-rite i1 Display Pro, ColorMunki Display, or Spyder 4 Elite or Pro (the Spyder 4 Express uses the same sensor, but the software is too limited to be useful).