If you have upgraded to Lightroom 4, when you look at photos in the Develop module that you had in a previous version of Lightroom, you will see an exclamation point in the bottom right:
This is a signal to you that the photo is continuing to use your settings from the old (pre-Lightroom 4) processing technology. The photo therefore should look the same to you as it did as you had left it in the earlier version of Lightroom (worked or unworked), and you will have the same Develop controls as before (for example, Recovery and Fill Light in the Basic panel, instead of Highlights, Shadows, Whites and Blacks.)
The new Lightroom 4 controls/technology are called Process Version 2012; the old Lightroom 3 controls/technology are called Process Version 2010. Both are built into Lightroom 4. New photos you import into Lightroom 4, or photos that you never worked on in Lightroom 3 automatically use Process Version 2012. Photos that you worked on in Lightroom 3, however, start out in the old Process Version 2010, and you must choose whether to update them.
Forgive me for the length of this post, but before I explain how to update, I need to first explain IF and WHEN I think it is appropriate to update.