Check your camera's picture style settings, which are only applied to the camera JPEG file. If this has changed the raw when viewed next to JPEG inside LR will look different.
The jpeg files I'm talking of don't come from the camera, they are derived from CAMERA-RAW files exported by Lighroom for further treatment with Photoshop CS6.
When importing theses "CS6 - retouched"files into Lightroom colour + contrast changes appear.
When exporting these jpeg files or older "CS6-retouched" jpeg files, that were previously correctly imported into LR4 (without changes), colour + contrast changes appear
There is no link with camera settings as i'm shooting CAMERA RAW files only,
I suggest uploading one of the JPEG files to Dropbox or other file sharing site and put the share link in a reply here. When you Export a raw file to JPEG inside LR does that JPEG file look correct BEFORE editing in PS or only AFTER editing it in PS?
If export a RAW file into High Res JPEG - it looks correct when I open it in PS
If I import this High Res JPEG file into LR, without processing it in PS, it looks correct
if I open this High Res JEG in PS, after processing it in PS, without import in LR, it looks correct
If I import this High Res JPEG file into LR, after processing it in PS, the colours and contrast are altered in the LR library
I hope you'll be able to understand what I mean
OK, this is a very simple issue. You're Exporting in LR to JPEG with Adobe RGB Color Space, but "opening" in PS in sRGB Color Space. To fix this you need to enable "Warnings" in PS. Here are the Color Settings I recommend using in PS. I created a 'Settings' preset' with the name 'Lightroom Workflow Warnings On.' Don't expect to see that.....you'll need to make the changes yourself.
When you open a file in PS with other than ProPhoto RGB color profile you'll get the below warning. You can then choose 'Use the embedded profile (instead of the working space).'
JPEG is a lossy file format and TIFF format is a better choice when the file is to be edited in PS. Here's the 'Edit in PS' settings I recommend for best results. Once you're finished with all of your PS and LR edits you can Export to a JPEG and delete the TIFF file to save disk space.