How to read Slide Switch schematics?
Those parts in the middle are rigid and are attached to the slider so they move together. They have 4 positions and will short various pins depending on the position. 



As you can see, the blacked-in dots that go to double contacts can be considered the common of each pole and they will each be connected to one of the four adjacent non-blacked-in dots (depending on the slider position), as in the below schematic, which I've arbitrarily numbered. You can see the four independent sections A, B, C and D. Positions are numbered 1 to 4 from left to right, consistent with the above schematic numbering.