sure you have a clean copy of the level editor on your hard disc.
Please note that it is highly recommended to enable Volumetric FX and Bump Mapping in the tomb4 setup.
I - Download the Audio Package and convert it to wav with the startme.exe
II - Decide if you want to download the TR4 files (easy to install, but bigger) or the TOM files (takes longer to install, but smaller)
TOM - Download the Wad Package and extract it into your graphics/wads folder, the btb06a.tom or btb06b.tom you downloaded has to be copied into that directory too. Start the converter (tom2pc) and convert the level you downloaded.
TR4 - Just copy the downloaded TR4 file into your data folder