I'm on something with 4.5, too. Mac on Mavericks, 6 GB RAM. Works decently enough for me - there's usually a few seconds of loading delay early on in every session, when I'm going for my first time pivoting/moving the scene view, but that sorts itself out and other than that it works fine.
There is something a fellow beta tester recommended to me (he hasn't outed himself so I can't give him due credit), which really helped when I had crashes at the start. Let me open another window and go look for his post...
(Passage of time)
Ah, here it is, in the private forums, from February the 4th. No idea if it's relevant at all, but most of my Unity birthing pains stopped after following this.
Unity /Edit / Preferences / Packages
There are 3 choices External / Custom & Internal. I have mine set on Internal.
So far there is nothing in Unity help to explain the different choices.
If you choose Custom it asks for a URL, so that would not be on your Mac.
If you choose External, I don't know
That's why I chose Internal, I'm assuming ( & we know what that makes me !!!!) that this option either keeps the packages & or saves them somehow within Unity or somewhere on your disk drive.
My thinking is ( & it gets a bit suspect at this time of night) that if you have the option on External, then somehow Unity is trying to communicate with this external source & when you had the problem & tried to close it & restart, that is was still communicating with it & hence the error.
Well that's my theory & I'm sticking to it till one of you "Brainiacs" out there tell me I'm full of it.