I don't think they have officially said they are making a new season, but it's highly unlikely that they wont. The first 3 shows were pretty popular, plus there is still several Novels to cover. The next one being "A Dancing Very Merry Christmas," and things really start to pick up with that one. There are several more after that too, and the series is still going. So I doubt they will stop making the anime anytime soon.
Oh, and if your itching for some more Full Metal Panic goodness, then don't fret, because Boku-Tachi is translating the Novels. If you just watched The Second Raid, then the next Novel would be a "A Dancing Very Merry Christmas." Then after that is "Continuing On My Own." Which by the way, was just finished being translated just a few days ago.
Oh ya, and here is the link to their site.
http://www.boku-tachi.net/