You are given a string containing only 4 kinds of characters 'Q', 'W', 'E' and 'R'. A string is said to be balanced if each of its characters appears n/4 times where n is the length of the string. Return the minimum length of the substring that can be replaced with any other string of the same length to make the original string s balanced. Return 0 if the string is already balanced. Example 1: Input: s = "QWER" Output: 0 Explanation: s is already balanced. Example 2: Input: s = "QQWE" Output: 1 Explanation: We need to replace a 'Q' to 'R', so that "RQWE" (or "QRWE") is balanced. Example 3: Input: s = "QQQW" Output: 2 Explanation: We can replace the first "QQ" to "ER". Example 4: Input: s = "QQQQ" Output: 3 Explanation: We can replace the last 3 'Q' to make s = "QWER". Constraints: 1 <= s.length <= 10^5 s.length is a multiple of 4 s contains only 'Q', 'W', 'E' and 'R'.