The "every 4 days" is not set in stone. Someone who wants to get pregnant more quickly might call it CD 26, another person may even say CD 27 to be absolutely SURE they are only getting in one attempt in fertile window. Most people would call CD 26 the 4 day pattern. (I pretty much consider CD 25 to be every 3 days, this is still not too shabby and some have done this)