Rohan labels the vertices of a 3 cm by 4 cm sheet of paper as P, Q, R, and S, in that order going clockwise, so that P Q is along the long side. Then, he folds the rectangle across P R as shown below to determine whether or not it is a line of symmetry. He finds it isn’t, and proceeds to measure the length QS to see by how much the fold was “inaccurate”. What length in centimeters should he obtain?