  Using continuous unitary transformations (CUT) we calculate the one-triplet
gap for the antiferromagnetic S=1/2 two-leg spin ladder with additional
four-spin exchange interactions in a high order series expansion about the
limit of isolated rungs. By applying a novel extrapolation technique we
calculate the transition line between the rung-singlet phase and a
spontaneously dimerized phase with dimers on the legs. Using this efficient
extrapolation technique we are able to analyze the crossover from strong rung
coupling to weakly coupled chains.
