  Four-spin exchange interaction has been raising intriguing questions
regarding the exotic phase transitions it induces in two-dimensional quantum
spin systems. In this context, we investigate the effects of a cyclic four-spin
exchange in the quasi-1D limit by considering a general N-leg spin ladder. We
show by means of a low-energy approach that, depending on its sign, this ring
exchange interaction can engender either a staggered or a uniform dimerization
from the conventional phases of spin ladders. The resulting quantum phase
transition is found to be described by the SU(2)_N conformal field theory. This
result, as well as the fractional value of the central charge at the
transition, is further confirmed by a large-scale numerical study performed by
means of Exact Diagonalization and Density Matrix Renormalization Group
approaches for N \le 4.
