Which of the linux distributive is more stable and based on 4.x branch? I using kubuntu, now. But it very 'buggy'. Or, such variant, I should to use alternative (ppa) repository?

If you're new to GNU/Linux you could try Mandriva.
Otherwise, Debian or Archlinux are very good, but not so "userfriendly" in the installation and initial configuration process...

After that, using KDE Desktop is almost the same everywhere.

You could try PClinuxOS.

openSUSE is also considered one of the better KDE-oriented distributions (although they give a lot of support to both KDE and Gnome).

As you can see, there are plenty of choices, and the best thing is that they are all good, imo. Some have certain issues, while another will have a different set of them. I am a Kubuntu fan myself but usually try others when something new is out. So distro shopping is always a good idea, and can even be fun .

As for Kubuntu and PPA's it does present you with options. I recommend most Kubuntu users to use the Kubuntu Updates PPA which will upgrade you from KDE 4.4.2 to 4.4.5. I did a blog on this which hopefully clarifies the ppa thing here