str_to_matrix.java

    public static void main(String[] args) {
        String source = "Hello world";

        char[][] matrix = toMatrix(source);

        printMatrix(matrix);
    }

    public static void printMatrix(char[][] matrix) {
        for (int i = 0; i < matrix.length; i++) {
            for (int j = 0; j < matrix.length; j++) {
                System.out.print(matrix[i][j] + " ");
            }
            System.out.println();
        }
    }

    public static char[][] toMatrix(String src) {
        char[][] chars = new char[4][4];

        for (int i = 0; i < src.length(); i++) {
            int row = i / 4;
            int col = i % 4;

            chars[row][col] = src.charAt(i);
        }

        return chars;
    }