The bit about being sick for four days in a row including non-working days is not the same as the rule about the three waiting days. The four days is about linking periods of sickness and a spell of sickness must last at least four days otherwise it does not count as a period of sickness. If you take an example of someone working only Mondays and Tuesdays, if they are sick for two weeks, they will not be paid SSP until the second Tuesday because their three waiting days will be Monday, Tuesday and the following Monday. If they then go off sick a month later for a week, that links with the previous spell and they had already served the three waiting days so the SSP starts immediately