So what did the story teach?

The story of the "Four Who Entered Paradise" teaches of the dangers of entering and returning from Paradise (Heaven). Such encounters could cause the ill prepared mind of man to basically short circuit. As an example, entering Heaven and becoming one with All, including God, Jesus, and a trillion souls, does not make you those you encounterd... you may feel to actually be Jesus, and others..... You may feel to have special gifts of the supernatural sort.... you may think yourself to be a "religeous" figure with grand delusions. Mystical travelers in Hassidic tradition have warned over and over again that visiting the Garden is extremely hazzardous...... and they are quite right. The entire point is that man's basicallly simple mind cannot handle the download of the concepts and information found within Paradise, Access should be limited to "those well grounded." The Yorde Merkavah (descenders to the throne) were some of the first serious travelers.... and even they would not venture Home without extensive guidance. These guys, as did Carl Jung in the 20th century, attempted to systemitize and compartmentalize Heaven. The bottom line is that the more they studied, the more they complicated the supposed "workings" of the "system" ...... they evolved not their understanding..... but rather they evolved confusion.
The early gnostics thought they had a handle on it, but the Serpent's Thorn was always in play. This is one of the reasons that the early Church shunned gnostic revelations.
In the 20th century, the the extreme example of danger began in Germany around 1918 with a gathering of characters then using peyote mixed with something called the Haggebotte Man (extremely toxic amanita muscaria). One of these characters, called Hister by Nostradamus, believed himself to be Jesus incarnate. The end result was the Holocaust.
In the 21st century, certain radical clerics, using a potion of the thorn apple (opium poppy used in the Temple of Morpheus) along with hashish, concluded that God had given them a mandate for Jihad (Holy War) against non believers. The Fools Gate to Heaven was thus opened.