This plugin allows you to have more than 1 EnvMap sprite per level. This is the reflection sprite, that you can control by editing the "Shine" or "Sheen" fields in StrPix or WadTool respectively. By default, TR4 can only have 1 individual sprite per level and it has to share with other effects.
Download the latest version. In TombIDE, click on "Manage Plugins," then select "Add Plugin from Archive."
Check out the newly added CUST_ENVMAP commands in the "Mnemonic Constants" tab of the Reference Browser.