What could/should we expect to decide to switch to 4.x ?
IIRC this was discussed sometime back on irc/wiki/mailing list/somewhere and the prevailing thought was that 4.x would happen when the solr implementation was stable. But then my memory may suffering from a parity error...
It seems that any "major" move forward would qualify for a major version increment, however. Something like DBIx, the completion of the move from C4:: to Koha::, the completion of the move from YUI to bootstrap, etc.