If it's between 4-8pm in Barcelona, then there are more than enough places to grab a cheap and delicious beverage. In a city that prides itself on affordable cocktails, 2 euro glasses of wine and overflowing pitchers of 'easy on the wallet' sangria, Barcelona doesn't lack when it comes to happy hour venues. Depending on what you're in the mood for, the city is your oyster. For those who enjoy mojitos and daiquiris, Milk Bar & Bistro serves 5 Euro cocktails daily. During the spring and summer months head towards the beach and check out some of the chiringuitos (beach side restaurants) all of whom compete to have the lowest happy hour drinks, many of which start at 2 euros a piece.
Many of the wine bars and slightly posher places in Barcelona have also jumped on the bandwagon and are serving half price glasses of vino tinto or 'two for ones' if you arrive between a very specific time. 4-6pm is usually a great time to check out happy hour specials as the bars and restaurants are usually empty, thus looking for ways to get more people into their locales.
Wherever you are in the city though, all you have to do is walk for a few blocks (or less) and you'll usually see a sign advertising great drink deals - as long as you show up at the right time.