The first win condition.

The checkWinCondition private method implements this rule by scanning whether or not the last move is a winning one:

... 
private Color winner;

public static final int DISCS_FOR_WIN = 4;
 
public void putDisc(int column) {
  ...
  if (numOfDiscs < ROWS) {
    board[column - 1][numOfDiscs] = currentPlayer;
    printBoard();
    checkWinCondition(column - 1, numOfDiscs);
    switchPlayer();
    ...
}
 
private void checkWinCondition(int col, int row) {
  Pattern winPattern = Pattern.compile(".*" +
    currentPlayer + "{" + DISCS_FOR_WIN + "}.*");

  // Vertical check
  StringJoiner stringJoiner = new StringJoiner("");
  for (int auxRow = 0; auxRow < ROWS; ++auxRow) {
    stringJoiner.add(board[col][auxRow].toString());
  }
  if (winPattern.matcher(stringJoiner.toString()).matches()) {
    winner = currentPlayer;
    System.out.println(currentPlayer + " wins");
  }
}
 
public boolean isFinished() {
  if (winner != null) return true;
  ...
}
...