The capital of West Germany from 1949 to 1990 and the official seat of government of united Germany from 1990 to 1999 (4)

I believe the answer is:

bonn

(Other definitions for bonn that I've seen before include "Rhine city" , "Capital of the former West Germany" , "W German capital 1949-90" , "German city" , "Former West German capital" .)

I've seen this clue in The Independent.

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