The John Rambo character, played three times by Sylvester Stallone in the 1980s, filled a specific niche. The brutish, lethal ex-soldier, betrayed by a country full of wishy-washy bureaucrats and politicians, was an antihero for conservatives — defying authority and following his own moral compass, Rambo was a symbol of how American strength, courage, and cunning (plus firepower) could make the world a better place. Two decades later, the fourth movie in the series picks up more or less where the others left off. Rambo is working as a surly snake-wrangler in Thailand when he’s approached by a small group of Christian missionaries seeking his help getting upriver into violence-ridden Myanmar. Reluctantly, he escorts them into the country — and, naturally, eventually ends up rescuing them from an exceedingly nasty predicament. Fans of down-and-dirty action may get a kick out of this — Rambo is outrageously gory, with arms, legs and even heads getting vaporized at machine-gun pace — but it’s pretty shallow stuff. The film may actually help raise Western awareness of Southeast Asian atrocities, though it’s not clear if Stallone thinks the U.S. should go to war there, too. Rambo’s Burmese adventure only seems to confirm his character’s world-weary pessimism. C+