Lightroom 4's process version 2012 has a revised set of controls. For the most part, this will simply make your photos look better! Here's a brief explanation of some key differences, straight from Adobe.
Reshared post from +Jan Kabili
Lightroom 4 users – are you wondering why you don't have Fill Light and Recovery sliders in Lightroom 4? Here's the answer straight from Adobe.
Google+: View post on Google+