White 4 is an approach
This was a standard idea from the days before komi go, exemplified by the Shusaku fuseki but certainly not limited to that pattern. If Black started on two 3-4 points, White would approach one of them at move 4, with some general aims of preventing enclosures, unbalancing the game, and playing influence-oriented 3-5 and 4-5 joseki. Black in return would try to play in a steady style without losing the dynamic edge of sente.
After komi came in, there were many more formations of three stones that White could accept Black constructing, so this idea was less used. It does persist, however, in diagonal patterns and when the approach is against a 3-5 point (for example see jabberwocks).