The way I see it is with early repayments means you have to to get those four heads in a row, rather than just getting one head.
With the coin toss analogy....I agree the chance of a head/tail is unaffected by the previous results. So if you are given 5 minutes to flip coins and you lose if you get just one 'tail", then If you do five flips, then you have five times the probability of getting a "tail" than if you had just one flip.
So with your $25 invested in harmoney for five years, if you have four early repayments and reinvest the money four times in notes of the same grade with the same default rate, then you will have had five chances of striking a defaulter within the five year period.