There may be a C routine that can be compiled into the 4gl code that can do this, but by default the only thing returned to the 4gl program from a system call is the exit status. What I have done in the past, for instance to return the PID of a processes started, such as another 4gl program, is have the system call store the pid ($!) in a file and load that into a temp table which can be selected from and displayed in the window. Right below the DISPLAY statement you can have a PROMPT statement to accept user input or open a form in the window with a FOR INPUT clause. If you don't already have Kathy Kipp's book, INFORMIX 4GL, you might see about getting it, she covers a lot of ground.
Hope this helps.
Justin