The 4 digit code you are looking for is
2734*3 = 8202
Right page :
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]
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
