It is not gone. It is available for Process Version 2003 and 2010 images. You can revert converted and new import images to the older process versions in the Camera Calibration Panel or the Settings>Process Menu. When you do the old Develop Basic Panel controls will appear. You should give the new tools a chance. They are far superior to the old Fill Light option at pulling in shadow detail.
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.