How do you change (4, -1) from rectangular to cylindrical coordinates between [0, 2π)?
I will assume you meant "polar coordinates" since cylindrical coordinates are 3-dimensional and you have only supplied 2-dimensional Cartesian coordinates.
From the graph:
Note that standard arctan functions will return a value which will need to be adjusted to compensate for the point being in the IV quadrant so
Depending upon the version used you will typically get