I came across an issue when I was trying to run Informix 4gl programs (7.50FC6X1) on an Informix 12.10FC1 engine.
The error I received was "Please install an INFORMIX-4GL Run Time Facility and run this program again".
The only thing I did was to upgrade Informix from 11.70FC7 to Informix 12.10FC1.
After working with tech support, I found out that there were some message files for 4gl that were not in Informix 12.10FC1.
Once I copied them from Informix 11.70FC7 to 12.10FC1, the 4gl programs ran.
Here are the files that need to be copied.