  The temperature dependence of the specific heat and spin susceptibility of
the spin ladders with cyclic four-spin interactions in the rung-singlet phase
is explored by making use of the transfer-matrix renormalization group method.
The values of spin gap are extracted from the specific heat and susceptibility,
respectively. It is found that for different relative strength between
interchain and intrachain interactions, the spin gap is approximately linear
with the cyclic four-spin interaction in the region far away from the critical
point. Furthermore, we show that the dispersion for the one-triplet magnon
branch can be obtained by numerically fitting on the partition function.
