It has been recently shown that quadruply lensed gravitational-wave (GW) events due to coalescing binaries can be localized to one or just a few galaxies, even in the absence of an electromagnetic counterpart. We discuss how this can be used to extract information on modified GW propagation, which is a crucial signature of modifications of gravity at cosmological scales. We show that, using quadruply lensed systems, it is possible to constrain the parameter \Xi_0 that characterizes modified GW propagation, without the need of imposing a prior on H_0. A LIGO/Virgo/Kagra network at target sensitivity might already get a significant measurement of \Xi_0, while a third generation GW detector such as the Einstein Telescope could reach a very interesting accuracy.
