Yes a flexible pass is opposed to continuous.

A 4 days in a month pass can be used on 4 days of your choice between (for example) 8th April and 7th May. No need to stick to calendar months either.

So to use the App and activate the corresponding travel days?

Yes you activate each travel day at any point, I do it at the station before boarding the first train of the day when I’m sure I’m not sick/the train isn’t cancelled, etc.

Have a look here : https://www.seat61.com/how-to-use-an-interrail-pass.htm → it’s well explained :)

(Also note that seat reservations cost extra and are sometimes required, for example on Eurostar trains. These ones in particular have a passholder quota and often sell out days or weeks in advance. If you wish to travel soon, book your Eurostar as soon as possible. Feel free to ask anything.)

If you travel in July for example you can book seats now?

Yes you don't have to activate the pass either.

Have a look at this for the best places to book: https://www.seat61.com/interrail-and-eurail-reservations.htm