Yes, they need to be lined up with each other
For the 4x4x4, you have to mine the FACE of the slab you want. So, 16 blocks. Mine a line, 4 up, and then 4 to the sides until you've mined it flat. Then, you wanna go ABOVE, usually I make ladders, and dig a tunnel above the slab I want. So, I place ladders to the left of the face I just mind (on the WALL to the left, not on the face itself), I mine 2 blocks up (because the face is 4 blocks tall, it's a total of 6 blocks high), the two blocks after the face is so I can walk through the tunnel- because you then need to dig TOWARDS the slab, mining 4 blocks forward. This would leave the slab to the right of you. So, you dig to the right, 4 blocks out of the tunnel. This would be the TOP of the slab.
So you've mined the face of the slab, and then a lil tunnel above the slab itself. Finally, you place 4 plugs in the floor of the tunnel, all lined up, and go back down and place 4 plugs on the LOWER blocks of the face. This would be the whole 4 (wide, part of what you mined for the face) x 4 (tall, also part of the face) x 4 (long, which is why you dug the tunnel outside of the slab, to reach the end of it, 4 blocks, and then put the plugs alongside, parallel to the plugs on the face)