This is a bugfix release of Tiki 4.0, released a month ago. This includes over 150 bug fixes, and minor enhancements.
We are tw:DogFooding Tiki4 for all the main *.tikiwiki.org sites.
The big question: "When should I upgrade?"
Tiki3 has been identified as Long Term Support. If you have all the features you need, you can stay with Tiki 3.x for a while.
Otherwise, you should upgrade to Tiki 4.x, which has more features and a better user interface.
Tiki5 is already well underway!