If that is your only train travel then first place is to look for the price of regular tickets.

https://www.renfe.com/es/en

Just like most other modes, the prices are flexible so what you see today may change.

RENFE offer their own pass, this is valid for a specific number of journeys, 4, 6, 8 or 10 over a period of 1 month, available in either standard or comfort (premium)

https://www.renfe.com/es/en/viajar/prepare-your-trip/abonos-ave-y-largadistancia/renfe-spain-pass

4 journey in comfort is €270.

Once bought you can then book seats online with no extra charge, a facility to change the booking is also available.

An Eurail one country-Spain pass is €263 for 4 days in 1 month, this gives unlimited travel on each day selected but there is a reservation fee of €10 on all long-distance trains in Spain.

As well as that there is an inability to book reservations for these online, booking from a Spanish train station counter is the only method, You can also “pre-book” by phone no more than 3 days before you are able to collect from a Spanish ticket office. This will hold the space but lapse if not collected within the 3 days.

https://www.eurail.com/en/eurail-passes/one-country-pass/spain#/

Do your calculations and decide which fits your plans best.

For Marbella you have to go to Malaga and then transfer to a bus service (Or train to Fuengirola then bus).

OOI there are 2 routes from Seville - direct through the hills on a slow train (about 3 hours and very scenic) or via Cordoba on AVEs. (about 2 hours depending on connections). The Spanish rail pass would count the latter as 2 trips of your allowance, Interrail uses travel days so both trains would be included but 2 res fees needed.

For the higher RENFE train class you need a first class pass. Reservation fees vary between trains and cabin chosen. For first class the fees are high when they include refreshments. 

https://www.interrail.eu/en/plan-your-trip/tips-and-tricks/trains-europe/high-speed-trains/ave