Informix runs on many platforms, and MSSQL only runs on Microsoft Windows.
The Informix 4GL is pretty tightly integrated into the Informix Database product. I can't remember seeing the Informix 4GL used without the Database Engine.
If you are running Informix on the Windows platform, create an ODBC entry for the SQL Server and things ought to work well. If you are running on one of the *ix platforms, create a JDBC connector (the mechanics are somewhat platform specific). If you are running Informix on the AS/400, create an Informix Data Connector. If you are running z/OS, ask your systems programmer for assistance because that requires specific knowledge of your configuration.
In theory, theory and practice are identical. In practice, theory and practice are unrelated.