I have been dreading this week ever since I started couch to 5k a few weeks ago. Up to this point, the running had been at most a combined 9 minutes over a total workout of 28 minutes. I would sometimes throw an extra minute or two of running in there because I felt weird walking for 8 minutes at the end of a workout in week 3, but overall the amount of time walking was much longer than the amount of time jogging. Another advantage to the first three weeks is that the walking legs are equal to or even greater than the jog that just proceeded it.
In week 4 both of these things change and it feels a little like you are being thrown into the deep end after slowly building a little momentum. The total for jogging in week 4 is now 16 minutes - two legs each of 3 minutes and two legs each of 5 minutes. Two weeks ago I wasn't even sure I could run for 3 minutes straight and suddenly I needed to run for 5 minutes, twice. For me the walking legs were actually a harder change because now they were half of the previous jog, so the 3 minute legs were followed by 1.5 minutes of walk and the 3 minute legs were followed by 2.5 minutes. The total jog time to walk time is now 16 minutes of jogging to 15 minutes of walking, including the warmup and cool down.