There is no '4' element in $array for you to access. $array is the string "4601". It is not an array (or an array reference), exactly like the error message you quoted says:
in reply to Re^2: how to find the how many elements are present in an array
in thread how to access each element in an array?
Can't use string ("4601") as an ARRAY ref
Because it is not an array, it has no elements for you to access.
Perhaps you're coming from C and assuming that strings are arrays of characters? If so, then you need to understand that they aren't. In Perl, a string is a basic data type. A single value. Not an array of characters.