Using a DB rail pass

I don't know about some E-version of the GRP. I do know that handwritten errors can void a normal pass. If you have any doubts at all about how to proceed with it, I'd advise you to stop in at the German Rail Pass Aid Office at your airport to clarify. In fact, I think it's wise to BUY THE PASS THERE so that you can get advice in person and ask any questions.

https://assets.static-bahn.de/dam/jcr:ad222c00-70a1-4159-95c6-3fbd25af917a/240076-319159.pdf

Reservation signage indicates the travel leg. Let's say you travel from Frankfurt Flugh to Munich but the seat you want has a reservation note that says Nürnberg > Munich... then you can sit there as far as Nuremberg, and move to some other unreserved seat when people get off in Nuremberg.

Bathrooms yes. Food not necessarily, and usually not much good. I pick up prepared food and drink at the station if I expect to be hungry. The Bord-restaurant tables tend to be occupied by people just hanging out. It's more comfortable to eat where you're seated. You can order from your seat if you wish to try the food, however.

You can't always hear the announcements clearly, even in German. Pay attention to the digital messaging at the end of each car.

You can use the S-Bahn network in those cities but not the U-Bahn (underground.) But keep in mind that your pass is only valid on the 4 days (same days you are traveling by train, presumably.)

https://sbahn.berlin/fileadmin/user_upload/Liniennetz/S_U-Bahn-Liniennetz_Innenstadt.pdf
https://www.vgn.de/liniennetze/schienennetz_s_bahn/
https://assets.static-bahn.de/dam/jcr:209ea092-1ea7-4f5c-817f-5d42aa2e96e5/204822-275325.pdf

I haven't used a German Rail Pass in over 20 years, so you probably need to verify this information, but I don't think some things have changed much.

I will be using the local rail/subway which is NOT part of the DB pass?

Local trains (IRE/RE/RB and S-Bahn) are trains of the Bahn and should be included with the pass, even in cities. The S-Bahn is underground in city centers so some people mistakenly think they are "subways". S-Bahn are commuter trains, like the Long Island Railway. Local transportation (U-Bahn, buses, and streetcars) are run by local transport companies and would not be included with the pass.

It looks like you are using a flexi-pass (not a continuous pass valid each consecutive day until used up). There has to be a way to indicated before boarding the train that you are using the pass that day. On paper passes you write in the date before boarding your first train that day. For an electronic ticket, there must be a way to do the same thing. (On a paper pass, care must be taken when filling in the date to be sure to write numbers correctly in the European manner. Hint: their one looks like our seven; their seven looks different - crossbar - than ours.)

Read the rail pass 'Conditions of Use'. download pdf here: https://assets.static-bahn.de/dam/jcr:004f4965-b323-4852-aee6-3fa77fba32ec/German_Rail_Pass_2022.pdf

5) If a Paper-Pass is issued “open date”, the
passenger chooses the first and last day
of travel and this must be inserted by
the Issuing Office or by a DB Ticket Office
before the passenger starts with the
first trip. A ticket inspector may also
validate the Paper-Pass when the holder
arrives in Germany on a train from abroad.
The passenger must not fill in the
validity dates him/herself.
6) Holders of a FLEXI Pass are free to choose
the days of travel and must enter the travel
day in the calendar box on the Paper-Pass
with an unerasable ball-point pen before
boarding the first train on that day.

FYI I ordered a rail pass via their online system. Although I specified a 30-day validity period, they failed to put that on the ticket. Because of that, my ticket must be validated at a rail center, as one of your earlier responders stated. Hopefully you'll be luckier. My arrival airport (Dresden) does not have a DB Zentrum in the airport.
-- Ethan