has 3 positions a, b, c

has 3 positions d, e, f

has 3 positions g, h, i

has 3 positions j, k, l

As each of the 4 knobs can be in any of the 3 positions,

the position combinations are....

adgj, adgk, adgl,

adhj, adhk, adhl,

adij, adik, adil,

aegj, aegk, aegl,

aehj, aehk, aehl,

aeij, aeik, aeil,

afgj, afgk, afgl,

afhj, afhk, afhl,

afij, afik, afil,

and so on.

Or.... first knob has 3 possible positions.

For each of these 3, the second knob also can have 3 positions.

The third one can be in any of 3 positions for each of the 2nd knob's positions. The 4th one can be in any of 3 positions for each of the 3rd one's 3 positions.

This is combinations