I'm seeing two kinds of 4x4 ceiling tiles from Armstrong - drop type and metal concealed hung. I can't find anything specifically regarding lights and the drop tiles, but based on their description of them as highly reflective for lighting purposes and the fact that none of their product photos show them with lighting installed in the tiles, I'm guessing they may not be rated for fixture support of any kind. All the photos I saw on
this page show a border around the tiled sections of ceiling where lighting and exit signs would go. Sounds like you need to make a few calls.