Ungefähr, etwa, and circa all mean the same, they are interchangeable. In my opinion, the only difference is that "etwa" sounds a bit too colloquial to be used in a very professional context (like a scientific research report or so). But that doesn't mean that the other two are very formal, you could use all three of them in any normal conversation.
"so gegen" has a similar meaning, but is only used when talking about time. It means "at around", as in "wir treffen uns so gegen zwei" = "we are meeting at around two". But while "around" could mean a few minutes before or after two, "gegen" literally means "against", so it indicates that you will meet no later than two, but possibly a bit earlier.
Hope this helps :)
Liebe Grüße, Jessica