That is one hell of a jump from 3.1.11 to 5.0. Looks like they are trying to align the version numbers.
Yep there were no 4 versions, iv been watching the 5 beta cycle which was fairly short in comparison to the firefox fiasco, dont really know why they bothered to release 3.1.11 since it was so close to this release though other than to inconvenience everyone with another download.
If thunderbird starts to take on the updating frequency of firefox it is surely going to keep the mozilla users busy and probably a bit pissed off.