Yeah, something is better than nothing. Even 4" to 6" is only starting to roll off the lower bass, but it's having plenty of effect on taming reverb of the highs. That said, thinner panels can be used at the first-reflection points along the walls, while thicker ones are placed in the corners.
The first reflection-point can be found by two people. One sitting in the listening position. The other taking a flat mirror along the wall. The "listener" only turns his head to the left and right, while the mirror is moved forward and back along the wall. When the speaker comes into view in the mirror, that's where to place a panel. Also between the speakers in the front of the room, and along the rear wall.
For the corners, if you can afford it. Take enough rock wool panels to stack to the ceiling. Then you cut each 2'x4' panel in half so you have two 2' squares. You divide those squares diagonally so you have triangles with 2' on each side (and a hypotenuse which Pythagoras could tell you the length of). That'll give you four piles of triangles each big enough to reach the ceiling in the four corners of the room. That'll provide excellent bass trapping.
