I will buy a French-Spain 4 day rail pass (its cheaper than Pt-Pt. Can you purchase the seat reservations on line, in advance?
Yes, we recommend that you do book reservations online for the TGV and AVE trains at the same time that you buy the rail pass, since the number of places for pass holders is limited. Reservations are typically available starting 2 - 3 months in advance of your travel dates.
Since you have to get your reservations so far in advance when using the high speed TGV trains in France, you can easily find pretty cheap advance PREMs and idTGV fares which I have usually found are much cheaper than pass days in many cases.
Did you check PREMs and idTGV prices some time in the future? Both of those include seat reservations at no extra charge.
I don't want to dissuade you from buying a pass if that is what you want to do - - but I would like to be sure that you have all the facts.
A lot of people in your situation rely upon figures from Rail Europe and the Rick Steves rail price map - both of which will probably quote you much higher prices than you need to pay.