I dont think that using only 4 vanes means that the missile has a single engine... every vane could control one engine from the 4 to steer the missile.. But this is just an opinion
I think youre right, using four vane for each nozzle increases cost and makes control of them more complicated.
This missile is massive...compare it to the russian ones which have much bigger range and are much smaller.why is that?
the fuel and material.
four vanes in Russian rocket: