We (my Serbian friend and I) are going to study in Cádiz for a month starting August 1st, but have 4 days in Andalucia before the course starts. We want to stay in youth hostels relatively nearby, and our options are spending the whole 4 days in Seville or dividing it 2 days Seville 2 days Huelva or 2 days Seville 2 days Chipiona. Which option would you recommend?

Otherwise we'd definitely go for the whole 4 days in Seville, but it's late July and I'm from Finland, so I might not be able to handle weather that hot with no possibility to swim for that long

You'll be hard-pressed to find lodging along the Costa de la Luz at that time of year. Peak Summer tourist season begins with la Virgen de Carmen (16 Jul). In Cadiz you'll be close to Chipiona so you might go a bit further up the coast, like to Sanlucar de Barrameda where the Manzanilla comes from. I think I would stay on the coast maybe in Mazagon (Huelva) or one of the smaller beach resorts between there and Matalascañas along what they call the Costa Ballena.

Stay close to the water!

I recommend you not to stay in Sevilla at weekends in summer. Everybody goes outside, to the beach or “a la sierra”, and it becomes a ghost city on Sundays. Huelva city is 2/3 hours by bus from Cádiz city and it has no beach, so it’s better you visit Chipiona, just 60 kms from Cádiz and well comunicated to Jerez (hourly buses), Cádiz and Sevilla.
Although there are many hostales and hotels in Chipiona, you will need to book before. This town has lots of people in August. Check

www.chipiona.net
http://www.chipiona.org/ciudad/alojamientos.htm

As Eddie says, Sanlúcar is a good option but the beach is not so good. And don’t forget to be close to the water!! It is usually hot.

Huelva is one of the ugliest cities in Spain (my apologies to all the "onubenses", but they will probably agree). The city itself has no beach, but 16 km away there is a town called Mazagon, with a huge wide beach that goes all the way to the National Park "El Coto de Doñana", more than 50 km long. However, if I was you I would stay in Sevilla and get to know the city. It has a lot to see, and later on, when you are in Cádiz it's gonna be easy to go to Chipiona or Coto de Doñana.

The problem we have with Mazagon is that you can't book the albergue there in advance (or at least they replied to my email that I should try booking "más adelante") and it'd be nice to arrive just to see there's no space left; our acommodation budjet is pretty limited

And as I said, I'm scared that in Sevilla it will be too hot for us to be able to fully enjoy it.

try the Jerez Youth Hostel, by staying there you could do day trips to nearby beaches such as Chipiona, Puerto de Santa Maria, Conil, etc. all well communicated by bus from Jerez..