Petr Pisar <ppisar@redhat.com> schrieb am Fri Nov 14 2014 at 2:46:02 PM:
On 2014-11-14, Thomas Spura <tomspur@fedoraproject.org> wrote:
> I'll update the zeromq package to version 4 next week in rawhide, which
> will break all existing packages that depend on the current zeromq package
> as there have been quite some changes [1].
[...]
> Similarly, I'd like to retire the zeromq3 package,

Is zeromq4 protocol compatible with zeromq3? And with zeromq2? Or are
they backward compatible?

According to [3], the protocol is extended so the old one should be compatible.
Between version 2 and 3, the functions were renamed/changed, so that will be the major difference. IIUC version 4 added security features, but the rest should be compatible with version 3 [4].

Greetings,
    Tom

[1] http://zeromq.org/docs:3-1-upgrade
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1145100
[3] http://zeromq.org/intro:get-the-software
[4] http://rfc.zeromq.org/spec:23