which 4gl compiler

We have an application (web) that either was compiled with a 4gl tool completely or some part of it. I am not sure about that. The company who created it is no longer in business. We will need to modify parts of that application soon. I intend to learn some 4gl soon. How can I identify what tool the company that made it, used for 4gl? And which one would you recommend for me to use? Is it possible to reverse engineer 4gl files already compiled? I appreciate every suggestion as I am very new to all of this (2 weeks ago I did not know what 4gl was at all). Thank you all, Gentian