Need Code on Masking the letter in a word from the input file using Unix Shell Script ( Charges are applicable )

Masking the letter in a word from the input file. Consider an input file consisting of bunch of words in a new line with word length ranging between 2 and above. Write a code to mask all the letters after first four letters in each word with # symbol as shown in example below.

Example: 

Input: ISSStudentsAreBest

Output: ISSS##############

Input File (input.txt)

After Executing Script.

Output File (output.txt)

If interest or Need Unix Shell Script or guidance on Masking the letter after fourth letters based on input file as shown above. 

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