I have documented the lighter print output from LR 4 a while back and have been waiting for someone at Adobe to notice and offer some solution to this problem. I use a robust printing workflow and have done the experiments under very carefully controlled conditions. The same photograph printed from PS CS5, PS CS6, LR3 all look the same and the print from LR4 is a good deal lighter, probably 1.5-2 stops in exposure terms. This is a problem that seems specific to LR4 and has persisted over several updates. Could someone from Adobe address this question please.
Cemal Ekin
Posts documenting printing problems:
http://www.keptlight.com/2012/03/lightroom-4-printing-issues/