J.P. Vigier, Brad Larsen and Steve McCarthy also scored for Atlanta.
Jason Blake scored the only goal for New York, which had a three-game winning streak snapped. Rick DiPietro made 15 saves on a night he struggled in the nets.
The Thrashers finished a stretch in which they played seven of eight on the road. They went 4-1-2 in that stretch and are tied with Buffalo for the Eastern Conference lead. Both teams have 23 points.
Atlanta took a 1-0 lead just 2:59 in when Vigier found a loose puck during a scramble in front and flipped a backhanded shot over DiPietro.
The Thrashers scored twice in a 69-second span late in the period to go ahead 3-0.
Hossa was the beneficiary of a poor clearing attempt by DiPietro. The puck was picked off the left wall by Niko Kapanen, who quickly fired a shot that was tipped in by Hossa at 17:18. His 12th goal moved him into a tie with New York Rangers forward Brendan Shanahan for the NHL lead.
Larsen made it 3-0 at 18:27 by redirecting a shot that DiPietro appeared ready to catch.
Blake got the Islanders on the board with his team-leading eighth goal at 5:13 of the second period. His right circle wrist shot hit the crossbar and bounced off Hedberg's back and into the net.
Alexei Yashin's secondary assist on Blake's goal gave him a team-leading 12 assists and 17 points. Yashin has points in four straight games (two goals, four assists), and six of seven (three goals, 10 assists).
McCarthy scored with 1:02 left in the period on a power play rebound to make it 4-1.
Thrashers forward Scott Mellanby was injured at 7:41 of the third period after he was checked into the boards by Yashin. Mellanby appeared to hurt his left shoulder or arm.
New York lost defenceman Tom Poti at 10:27 when he was inadvertently elbowed in the face.
Notes: After 16 games last season, Atlanta was 6-9-1. ... The Thrashers are second in the league in goals scored with 57, trailing Buffalo by two. ... Islanders television colour commentator Billy Jaffe worked for the Thrashers on radio and TV the previous six seasons.