I have a random string X of 100 letters in which each letter is equally likely to be any 1 of the 26 in the alphabet. What is the expected value of the number of times the substring 'ABCD' occurs in X? Also, is the expected value the same for every substring of length 4?

Also, what is the expected value for the substring 'AAAA'? Since it can overlap, would the expected value be greater?