Well, I suppose it might be useful for you to know that Barcelona is part of the Catalunya region of Spain and the official language there is Catalan, NOT Spanish. Yes, people in Barcelona speak Spanish for sure. Quite well for the most part, but the Catalunyan government is strongly against the use of Spanish in the region. In fact, the last time I looked, their official government website was in 2 languages - English and Catalan. Some of the strong feelings against the Spanish language stem from the official repression of Catalan under Franco, but he's been dead for about 33 years now and they are still angry about it.
Why did I mention this? Well, I'm not sure that Barcelona is really the best place to go to learn Spanish because its in Catalunya. I think you'd do better to learn in Andalusia somewhere. The costs would be less and Andalusian Spanish is a bit closer to Latin American Spanish (if you care about that sort of thing) than other regional dialects. I was in Valencia, which is also in Catlunya but quite a bit south of Barcelona, and the local accents there were horrible. I guess it was from the strong influence of Catalan, but I had tremendous difficulties understanding people I talked to who were natives of the area. So as long as you don't pick up some kind of weird Catalan accent from studying in Barcelona, I guess it would be OK, but Andalusia would be better in my opinion. Barcelona is a very expensive city to live in, so I hope you are prepared for that.
Can you learn enough in 4 weeks to get by? Quite possibly, if you work hard at it. The good news for you is that Spanish is one of the easiest languages on the planet to learn. That's actually a good thing. The grammar is very very simple and exceptions are rare. Words are pronounced exactly like they are spelled. The only difficulty is that speakers of Spanish typically speak very fast, but eventually you can come to grips with it. People in Spain pronounce their words correctly, which will help you, unlike certain Latin American groups who always drop letters and sound like they have a potato in their mouths when they talk.
