Can anybody give a short summary of whats likely different between the 3rd and the 4th Era? Like, any new technologies or new races or new important persons? Is it like when we went from Iron Age to Bronze Age?
We don't know yet. It's possible that there will be an Interregnum lasting years, possibly generations. The Keyes novel coming out in November is set 40 years after the Oblivion Crisis.
I once theorized that the Interregnum could be similar to Japan's Sengoku Jidai period, but it may be that the Interregnum won't last as long as Japan's warring states period. http://en.wikipedia.org/wiki/Sengoku_jidai
I don't expect new technologies, but that's only my opinion. If I were Bethesda, I would write the end of the Third Era to resemble the fall of the Roman Empire, complete with the equivalent of the Romano-Britons trying to pick up the pieces. http://en.wikipedia.org/wiki/Romano-British_culture
But as always, I'm no loremaster or historian.