The 4 digit code you are looking for is 
>!2734*3 = **8202**

Right page : credits to @Arturo Vial Arqueros for this part
>!You can link the top row and the bottom row of the letter bricks as stated on the right page, and reverse them as shown by the first teeth of the skeleton to obtain 2734 x [brown trapeze]
>! [![enter image description here][1]][1]

Left page :
>! We just need to figure what the brown trapeze (the last tooth) is worth. You can place the unused bricks to obtain a 3
>! [![enter image description here][2]][2]


  [1]: https://i.sstatic.net/tKUEb.png
  [2]: https://i.sstatic.net/EvLzf.png