Besides Epsom Salts this is the only cleanse I use. I have never fasted with it and never read about it either. Of course things might have changed, but I must have been one of the first ones that bought it when it came out. Everybody at that time was using colosan, but you had to mix it and oxy came in capsule form, easier.
You better read the directions again.
It used to be that they had you start with 10 capsules but I know they start you now slower. There is really no need to fast.

I just read the bottle and it does not say anything about fasting.
Since you are so nervous start on the week-end so you are close to a bathroom the next day. The old fecal matter will become liquefied (water) so when you have to go make sure you are at home. If you are worried start with less, but if you can be home do the 4 and on the days you are out do some less and more on the week-end.
They use to start with 10 capsules but they changed it so for some people it was easier to start slowly. I am the opposite I jump in and want to get it over with.
I would never be able to do a fibercleanse and do it for months. One tried it for 4 days and stopped.