print random char without repeat in square?

I'm generating random characters via my code below:

static void printRandomWhatEver(int NUMBER_OF_CHARACTER,
                                int NUMBER_OF_LINES,
                                char char1, char char2) {
    int DIFFERENCE_SQUARE = 4;
    char oldChar, newChar, random;
    for (int i = 0; i < NUMBER_OF_CHARACTER; i++) {
        random = getRandomCharacter(char1, char2);
        if ((i + 1) % NUMBER_OF_LINES == 0) { // new line each new NUMBER_OF_LINE
            System.out.println(random);
        } else {
            System.out.print(random);
        }
    }

}

My getReandomCharacter:-

private static char getRandomCharacter(char ch1, char ch2) {
    return (char) (ch1 + Math.random() * (ch2 - ch1 + 1));
}

All working fine, what i need is the chars inside square 4*4 numbers to not be similar, here is image to explain what i need.

The digit 4 is in the same square:-

**
**

How i can prevent digits to be similar in a part of square in the printed digits or chars?

Thanks in advance.