If that’s the only trip you will be making: book your tickets (reservation included) here: https://www.sncf-connect.com/en-en/
Buying a pass for this, will be very bad value for money otherwise.
Best book soon, it’s close to easter so a lot of people will be travelling and there are only two direct TGVs per day.
Normally a pass is poor value for France and Spain, and definitely for a single journey. But prices for 4 April are pretty high.
By my calculations, you could buy a ‘4 days in a month’ pass + the necessary €37 reservation on the Paris-Barclona train for slightly more than the cost of a one-way ticket. This would be pointless if that is your only journey, but could be good value if you will have any other train travel (preferably not in France or Spain!) in the month.
But if you want to keep things simple, just buy a straight one-way ticket
If that's the only train you're taking, regular tickets would be best.
Indeed this journey is extremely expensive (due to the fact that there are only 2 TGVs per day, thanks SNCF)… 169-209€ per person
If you don't mind travelling a bit longer (and see more scenery of the Mediterranean), it can be way cheaper :
08:23 OUIGO TGV Paris Gare de Lyon - Perpignan 13:24
14:55 Perpignan - Portbou 15:47
16:05 Portbou - Barcelona Passeig de Gracia (conveniently located in the city centre) 18:34
64,20€ Paris - Portbou + 13,50€ Port-Bou - Barcelona
Book ASAP whichever option you choose !