1. at about.
The OED illustrates this use (e.g. at about seven o'clock in the evening) with examples from 1843 onward, and occasional objections must now be set aside.
2. at all.
In Standard BrE, this prepositional phrase meaning ‘in every way, in any way’ is restricted to negative constructions, questions, and conditional statements: I did not speak at all / Did you speak at all? / If you spoke at all. Its earlier use meaning ‘of all, altogether’ survives in Ireland and in some BrE and AmE dialects (e.g.

• John Cusack is the finest dancer at all —P. W. Joyce, 1910).

3. at or in.
It is more usual to use in when permanent location or continued habitation is involved, and is required when the place is a country or region rather than a place such as a town or city: Timbuktu is in Mali / He lives in Helsinki / The festival takes place in Salzburg in August / She grew up in Switzerland. At is more usual with reference to more transitory association, and is much more common with specific places: The plane landed at Nadi in Fiji / There is a railway station at Leuchars. A further distinction is demonstrated by the sentences They are at St Andrews [= a member of the University] and They are in St Andrews [living in or visiting St Andrews].
4. where it's at.
This colloquial expression, meaning ‘the fashionable scene or area of activity’, swept into AmE in the 1960s and is now common in BrE and other varieties. It should be avoided in more formal English.

Modern English usage. 2014.