[Sunhelp] Adding a "*" character to specific lines in a data file

[Sampath K Kari]

Dear experts,

I am very beginner in scripting.

Need ur help for a script that could do below:

Input data file is a flat text file containing data in columns delimited
by space.

I need to insert a "*" character in the beginning of every 4th line
starting from line 1 in a file.  That means I need to insert in lines 1,
5, 9,13, 17,  etc,, till the end of the file.

Thanks in advance.

-Sampath