Your original for-loop probably was incorrect. And why do you want a while loop? You already know how many iterations you want.
In this case you can avoid the loop entirely:
C(:,4)=sin( C(:,3) ./ sin( C(:,2) ) );
And are you sure you mean this, and not this?
C(:,4)=sin( C(:,3) ) ./ sin( C(:,2) );