This is the first win condition requirement for players.

In fact, this requires one single check. If the current inserted disc connects other three discs in a vertical line, the current player wins the game:

@Test
public void when4VerticalDiscsAreConnectedThenPlayerWins() {
  for (int row = 0; row < 3; row++) {
    tested.putDiscInColumn(1); // R
    tested.putDiscInColumn(2); // G
  }
  assertThat(tested.getWinner(), isEmptyString());
  tested.putDiscInColumn(1); // R
  assertThat(tested.getWinner(), is("R"));
}

There are a couple of changes to the putDiscInColumn method. Also, a new method called checkWinner has been created:

private static final int DISCS_TO_WIN = 4;

private String winner = "";

private void checkWinner(int row, int column) {
  if (winner.isEmpty()) {
    String colour = board[row][column];
    Pattern winPattern =
      Pattern.compile(".*" + colour + "{" +
           DISCS_TO_WIN + "}.*");
 
    String vertical = IntStream
                       .range(0, ROWS)
                       .mapToObj(r -> board[r][column])
                       .reduce(String::concat).get();
    if (winPattern.matcher(vertical).matches()) 
      winner = colour;
  }
}