Under which bridges do I have to fly to get the "Under the Radar" achievement in GTA4?
This site explains why you're having trouble.
This is the key exerpt:
Find the smokestack in Alderney’s industrial park. If you have maneuvered your helicopter underneath all four of the bridges that cross the inlet above Alderney’s industrial park and the game gives you credit for only three of the four bridges, fly to the bridge located at the opening of the inlet. Open the map and find the smokestack, which is indicated as a gray circle. Step 3 Maneuver the helicopter south of the smokestack. Fly your helicopter south of the smokestack, then fly straight ahead under this section of the bridge. Flying under the section that crosses water will not give you credit. Once you have flown under all 14 bridges, you will obtain the Under the Radar achievement and receive 40 gamer points.
Something I noticed when trying to get the achievement myself -- everytime you get one of the bridges towards the achievement it'll pop up a message saying how many bridges are left, and more importantly, the bridges you haven't flown under will temporarily get a trophy icon on the map. If you go to the pause menu, you can see all the bridges you have yet to fly under and create a map marker at the next bridge you want to go to. Note that the trophy icons only seem to last for about 10 seconds after flying under each bridge, when not on the pause menu.
If you don't know where to get started, all of the bridges that cross the various rivers around the islands are counted towards the achievement.
There are fourteen bridges, some more obvious than others:
If you pull up the map within a few seconds of passing a bridge, your map will briefly show trophies on the uncompleted bridges. Upon passing beneath each bridge without crashing, a text notification will show the count of bridges remaining. If the notification doesn't show, try the bridge in a different area. If you're missing one, there's a good chance it's the westernmost bridge—try passing near the smokestack instead of over the water.