4..0-0 5.a3 Bxc3+ 6.Qxc3 b6 7.Bg5 is nowadays the main line of the Classical Variation (although 6..b5!? is a gambit invented by Alvis Vitolinš).
4..Nc6 is the Zürich or Milner-Barry Variation (named after British chess player Stuart Milner-Barry).