Adobe changed the image processing algorithm in Lightroom 4.  The new algorithm is called Process Version 2012 or PV2012 for short. LR3′s version is PV2010 (and there’s still the original PV2003).  With this change comes the problem that Develop Presets created with PV2010 (LR3) may give you different results when applied to photos in LR4. Here is an article from Adobe’s blog about updating your LR3 Develop Presets for LR4 that explains what you have to do if you want to use your presets from LR3. Please read carefully and be aware about two things:

1. If you have imported an image with LR4 and you apply a preset that includes PV2010 you can not take advantage of the new features of PV2012 for that image, cause you just changed the process version back to pre-LR4.
2. If you want to use your “old” preset and take advantage of PV2012, you can apply the PV2010 preset, change the process version manually to PV2012 and then tweak until you are satisfied with the results and then save the preset under a new name.

Either way is not great, you might as well redo all of your presets. I wish Adobe would have found a way to make this easier for those of us who use Develop Presets extensively in their workflow.