Seasons Three and Four were made primarily because ITV commissioned the team for two more seasons after the second. This caused some problems distributing the series for a while since the final two seasons were thus the property of a different company.
God Does Not Own This World: According to writer and producer Martin Gates, this is why no revival projects have been made thus far. Creator Mike Jupp also had to step down as consultant and storyboard artist for the closing points of the series, due to being busy with future project Bimble's Bucket.
Keep Circulating the Tapes: All of six episodes were ever released on DVD, both volumes seem out of production. Even before that, only a selection of episodes from the first two seasons got a VHS release.
Not so much now that the series is available on Jaroo.com (but for how long is anyone's guess).
Still applies to series 3 and 4, which are owned by ITV rather than Cookie Jar, and are still unavailable.
The Other Darrin: Gary Martin replaced Leonard Whiting as Urpgor for the final season. He also took over narration duties Season Two onward.
What Could Have Been: The original story plans for the show were intended for a children's book and had a somewhat Darker and Edgier tone. In addition the story was to focus more on Rufus (who was a prophesised hero in early plans) with the Urpneys as minor comic relief. A few of the original concepts made their way into "The Dreamthief" promo and even the Opening Special, albeit in a Denser and Wackier tone.
Zordrak's original name was "Nasta Shelfim", an anagram for "Satan Himself". The name was changed in fear of offending religious groups.