Well, the Fill control of LR3 is not in LR4 in the same form, but the new controls can achive a similar effect - often better and without the artefacts.
- Very often, "Shadows" will do what you want.
- A closer equivalent to the old Fill is to increase Shadows and Exposure, and decrease Highlights slightly.
- Sometimes, to achieve the exact effect of Fill, you might also need to reduce contrast slightly and maybe increase clarity and saturation a little bit.
However, #1 or maybe #2 normally does the trick.
If you want an in-depth analysis of the new controls, see Rob's Cole's very detailed experiences at http://forums.adobe.com/thread/968940?tstart=0 and http://forums.adobe.com/thread/973706?tstart=0.
