Start by not using the app! (It’s not as helpful as you’d hope for route planning)

Have a look at the rail map of Europe to trace your route, then pick some places along the main lines you’d like to stop. Perpignan, Barcelona, Madrid would be roughly the route. You can also take the slower, more picturesque route through the Pyrenees at Latour de Carol.

Are you looking for suggestions of where to stop, or help with timetables?

Suggestions would be helpful for sure. I had kinda looked at the map and I had thought of going Perpignan , Barcelona then follow the coast line. The Pyrenees sounds interesting too. yes timetables would be very helpful. 
The main reason is can I stop at multiple places on each of my 4 day pass?
 

Yes, on each of the 4 days you can make as many train journeys as you like.

Bear in mind that in Spain and France the ‘hop on hop off’ aspect of the pass doesn’t really exist, because you need reservations for almost all the trains you’ll be taking.

(See here for the best guide to reservations https://www.seat61.com/interrail-and-eurail-reservations.htm)

Check out timetables by searching on Bahn.com. I don’t know when you’re travelling, but don’t search too far in the future - the timetables aren’t loaded very far ahead, especially for Spain. I would look at this month and assume the times won’t change that much in the future.

On my first day ,  I will travel from Toulouse to Barcelona and would like to stop at many places that I find interesting. 
when I bought the pass I thought I could do so at my pleasure but it sounds like I have to book all my train in advance. I have never been in Spain and not sure exactly where I will be stopping. 
any suggestions on how I can do my planning?

From Toulouse, you can take the TER up to Latour de Carol/ La Tor de Querol, then from there down towards Barcleona. Neither of those trains needs reservations, and you can hop on and off as you wish.

(Annoyingly, the timetables for the latter don’t appear in the Rail Planner or on Bahn.com, but the pass is valid - timetables on https://rodalies.gencat.cat/en/inici/index.html)

The same is true of the alternative route via the coast: Toulouse-Cerbère by TER, then local train from there to Barcelona - no reservations needed.

-

Most other trains in Spain, you should assume you need reservations. As above on the Seat61 link, you can get some, but not all of these online, or at any Spanish ticket office. 

I agree that it’s a bit of an unsatisfactory setup for spur of the moment travel.

None of the trans from Portbou to Barcelona need reservations. The rail planner app claims some of them do but it's incorrect.