The OFFICial rule is that you have to '’use'’ country of residence, whichy is where you have stayed the last 6 monthes and are registrated in the public administration. However, some countries do not even have such a registration system proper. But the rules were made for EURopeans and here anyone has.

EU rail as such is better value as INTERrail-because it does not have the '’only 1 trip OUT and IN'’ in your own country. Prices hardly differ.

Also double check if a pass is even worthwhile for just these trips-this will also depend on how many breaks you intend to make and/or side-excursion trips. There is now a direct overnight train from P to Wien=Vienna and for local travel, unavoidable with your plans, in FR and even more in ES-on TGV and AVE Hi-Speed trains, cumbersome and sometimes very hard to get and costing extra REServations need to be made.

The reservations can be quite expensive Reservations for Eurail & Interrail on Nightjet Paris - Vienna

Even the trains from/to Mardrid needs a reservation and as there is no direct train from Paris you need to change and because of that need two reservations

Paris - Barcelona (up to 56€on top just limited seats for Railpassuser)
Barcelona - Mardrid (usually 15-25€) 
same backwards.

There used to operate a TGV/AVE from Marseille to Mardrid
means Vienna - Frankfurt - Marseille in one day and then nextday Marseille to Mardrid.
Frankfurt - Marseille (17€ in 2nd Class)
Marseille - Mardrid (up to 56€ like Paris Barcelona)



 

Thanks for responding. In your opinion would it be cheaper to by the ticket from Paris to Vienna directly for the overnight train? - does it come to the same buying the Eurrail for 4 days and buy the extra reservation fee? Would really like to know what are you thinking about it. We never have used Eurrail before.