I have four installers built with separate .iss files: client installer, server installer, client update installer, server update installer. At the moment I use separate isl files for each installer.
My question is can I use Inno Setup preprocessor inside the isl file to decide which messages refer to which installer so I could use only one instance of isl file for each language?
You can use a preprocessor in .isl files, only if you use a hack like this one from your previous question: Full preprocessor support in Inno Setup language files (isl)
When you do, you can use /D switch on the commandline of the iscc process that preprocesses the .isl files (in the PreprocessLanguage macro). Use the variable to identify your build. And you can then use the variable in your .isl files any way you like. You already know how: How can I pass command line parameters with a value to the Inno Setup Compiler, so I can use them in my code?