Word is he originally wanted to end the series at 3 and too many people begged him to do more, so he made 4 as crappy as possible to get them to shut up, and it worked for a while and then eventually 5 came out of nowhere. Now there's no 6 and 7 is coming up.
The whole speech at the beginning seems to have been a subverted "take-that" at all the people begging for number 6.
Quite clever, really.