From the mailing list:<p><pre><code> For the first time in more than 25 years the major version number of
the software is changed. The decision to do this is based on the many
new features and new API. While backwards compatibility remains -
except in a few rare corner cases - the addition of a new and improved
programming interface warrants a new major release.
The new major version number unfortunately leaves the project in a bit
of a conundrum regarding the name. For the majority of the life-time
of the product it has been known as PROJ.4, but since we have now
reached version 5 the name is no longer aligned with the version
number.
Hence we have decided to decouple the name from the version number and
from this version and onwards the product will simply be called PROJ.
In recognition of the history of the software we are keeping PROJ.4 as
the *name of the organizing project*. The same project team also
produces the datum-grid package.</code></pre>