  A series of YBa_{2-x}Sr_xCu_4O_8 single crystals was measured to study the
influence of site disorder on the transition line Hss(T) between quasi-ordered
vortex lattice and highly disordered vortex glass, as well as on the maximum
critical current density within the glass phase, jcmax. When 32% of Ba is
replaced by Sr, jcmax is an order of magnitude higher than in the unsubstituted
compound. In contrast, the transition field Hss first drops by a factor of
about five with a substitution of just 10% of Sr for Ba, and then remains
approximately constant for higher Sr contents. Our results indicate that in
very clean systems the order-disorder transition is affected very strongly by
any crystallographic disorder, while above a certain threshold it is relatively
robust with respect to additional disorder. In all substituted crystals Hss
monotonically decreases with an increase of temperature.
