Hi. I’ve been trying to upgrade 3 pc from milestone 3 to 4 via internet but both yast and zypper freeze in various stages of the download. at reboot one of the pc was unusable. I was wondering if anyone could teach me the safest way to upgrade from one milstone to the next one…
I always re-install. I’m fairly sure most of us do. We want to test the installer as well you see.
Two weekends ago, I and others had a similar problem to you, while using the factory repos for updating during M3 (M1 & M2 upgrading/updating had been ok). I made the mistake of rebooting during a download freeze and suffered the unusable system. There are ways to avoid that, but it’s a bit late to get into that now, and it would need further detailed discussion. The cause of the problem was diagnosed by the developers, so I hope it hasn’t happened again! The Factory always has the potential to be an unstable environment.
In your case, I agree with @caf4926. Re-install using either the live-cd or DVD. Your original problems may also have involved an internet connection problem, so make sure to double-check the downloaded .iso (md5, etc.) before committing the burn to media. 
You only need to reboot when the kernel freezes though it can even sometimes be recovered with Alt+SysRq
The command rpmdb --rebuilddb usually solves the problem
does for me so far