Brandon Roy scored 18 of his game-high 24 points in the fourth quarter — including a 10-foot bank shot with 39.2 seconds left for the game’s final points — to lift the Trail Blazers to an improbable 84-82 first-round playoff victory over the Dallas Mavericks Saturday at the Rose Garden. The win squares the best-of-seven series at two games apiece, with Game 5 Monday night in Dallas’ American Airlines Center. Portland trailed 64-41 late in third quarter after missing its first 15 shots of the third quarter. The Blazers trailed 67-49 entering the final period. It was the biggest fourth-quarter comeback in franchise playoff history.