I am planning to get to the circuit by train, and I’ve looked into the various tickets available. There are a couple of useful links:
Map of the city showing both Metro and Rodalies train stops: http://www.fgc.es/eng/planol.asp
Tickets from TMB: http://www.tmb.cat/en/bitllets-i-tarifes
Central Barcelona is in Zone 1
Barcelona Airport is in Zone 1
Montmelo station (beside the circuit) is in Zone 2D
A ticket allowing unlimited travel in 2 zones costs €10.95 per day. However, if you’re only planning on going to the circuit this is not the best option. The cheapest one seems to be the T-10 ticket, €18.40 buys you 10 journeys within Zone 1 + Zone 2.
The good thing about the “journeys” is that they do not need to correspond directly to the number of trains you take. If you take the metro to the main stop to get the train to the circuit, provided you get on the train to the circuit within 75 minutes of arriving at the main station it will count as just one journey for both the metro and Rodalies train.
This ticket allows you to travel to and from the airport as well, so if you are planning on going 4 days, the T-10 ticket should be enough to do this.
I got info from here: http://www.barcelona-tourist-guide.com/en/transport/barcelona-metro.html#metroticket – not my fault if I’m wrong, sorry :p