  Holes weakly doped into a four-leg \tj ladder bind in pairs. At dopings
exceeding a critical doping of $\delta_c\simeq {1/8}$ four hole clusters are
observed to form in DMRG calculations. The symmetry of the ground state
wavefunction does not change and we are able to reproduce this behavior
qualitatively with an effective bosonic model in which the four-leg ladder is
represented as two coupled two-leg ladders and hole-pairs are mapped on hard
core bosons moving along and between these ladders. At lower dopings,
$\delta<\delta_c$, a one dimensional bosonic representation for hole-pairs
works and allows us to calculate accurately the Luttinger liquid parameter
$\krho$, which takes the universal value $\krho=1$ as half-filling is
approached.
