included in the set and the other three are the white ones included in the set.
I believe the light gray one is used on step 8 on page 23. However, the contrast
between white and light gray in the instructions is not very good which makes
it hard to tell. The ones used on pages 25 and 26 seem more white to me, though.
Lastly, the LEGO replacement parts website also only lists three of this part
as coming in the set.