Just as we're about to enter the dog days of the summer movie seasona time for sequels no one demanded, lame-brained horror flicks, and one-dimensional family filmsalong comes Four Brothers to crank up the summer heat. This explosive action-drama is about four foster brothers (two white, two black) who reunite to avenge the brutal murder of their adoptive mother. But what seems like a black-and-white convenience store killing goes gray at the edges as the list of suspects grows longer and more complicated. Part mystery, part character study, part balls-out action flick, Four Brothers is John Singleton's best movie since his 1991 breakthrough Boyz n the Hood, although that's not really saying much. It's not perfect, though. The films tone wobbles from highly spirited to high tragedy, but Mark Wahlberg is an old pro at this kind of street opera, while newcomers Garrett Hedlund (Friday Night Lights) and Andre Benjamin give solid performances. The end of the summer season is fast approaching, but with Four Brothers hitting the theater, the movies haven't gone completely to the dogs.