JR East Flexible 4 day pass

I am looking at purchasing the JR East Flexible 4 day pass but I was wondering if someone could explain to me how it works!

When you buy it do you nominate the days you want to use it? Or do you take it to a JR office on each day that you want to count as one of the 4 days? If so, one of the days I want to use it I want to catch a train at about 6:30am, will this be too early for them to validate my pass? It will be at Sendagaya station in Tokyo so not a big station.

Thanks!

My understanding is that the pass will get stamped when you pass through the manned gate the first time on a travel day. Gates are manned from the begin of operation.

In any case, there is no reason to be worried.

Does that mean you can't make reservations on days before you travel, only on the day of travel?

If you're going to be within the JR East area, you're not going to need any reservations (or at least any reservations in advance).