Now that Flaming Fist is officially the military strength of Baldur's Gate, do they still active as a mercenary force ?
If not, is there any info in 4E as to what replaces Flaming Fist as the major mercenary force ?
I'm not even sure how mercenary they were a century before. They acted as the city garrison and that still holds true. I do feel they have mercenaries that venture out of Baldur's Gate and to other, closer locations and/or work as protectors of Baldur's Gate caravans and other interests.
4E Realms = Great Taste, Less Filling.
"If WotC were to put out a box of free money, people would still complain how it was folded."
They are still considered a mercenary company, albeit with granted powers of a constabulary and garrisoning force. The sheer size of Baldur's Gate requires it (it is the largest city in all the Realms as of1480 DR).
...also to answer your question. Adventuring companies largely take on all freelance merc work. If your looking for specifics, the Vilhon Mercenary Corps has grown significantly in the past 20 years or so and can be found all throughout the Realms; acting much like the Knights Templar of our own world during medieval times.