Its either or: IF you insist on paper you DO have to state exact start date when buying-the advant of mobile is indeed you can decide on that later. IF you-or perhaps a smarty teener in your family are used to using fones then really these assumed difficulties with it are able to overcome by 100.000s of people this last yr.

On that paper you also will have to write down-in your case thus 4x same-the actual trips you make. ON mobile this can be combined a little smarter.

 You need to decide the start date of the validity of the pass, that is the 1st day of that you have the possibility to use the pass but this does not have to be your first travel day,  With a flexi pass you can then use the pass on any days of your choice during the validity period as long as you have travel days left.. 

I just bought my second Global pass  - paper - and have not had to put a start date on it.  Last year I just put that on the day I got it verified/stamped.

That is only possible for Eurail passes, not for Interrail.

@juliadbau  Eurailers can actiave their Pass on several European Stations. However Interrail travelers have to select a specific startdate during the booking process. This is one of the few differences between Eurail & Interrail but the offers become more and more the same. 

I’m glad I am eligible for Eurail, then! thanks for the information.