This is the same win condition, but in a different direction.
If a player inserts a disc and connects more than three discs of his color in a straight horizontal line, then that player wins.
A few lines to implement this rule are as follows:
... private void checkWinCondition(int col, int row) { ... // Horizontal check stringJoiner = new StringJoiner(""); for (int column = 0; column < COLUMNS; ++column) { stringJoiner.add(board[column][row].toString()); } if (winPattern.matcher(stringJoiner.toString()).matches()) { winner = currentPlayer; System.out.println(currentPlayer + " wins"); return; } ... } ...