  Four-quark states may exist as colorless meson-meson molecules or compact
systems with two-body colored components. We derive an analytical procedure to
expand an arbitrary four--quark wave function in terms of nonorthogonal color
singlet--singlet vectors. Using this expansion we develop the necessary
formalism to evaluate the probability of physical components with an arbitrary
four-quark wave function. Its application to characterize bound and unbound
four--quark states as meson-meson, molecular or compact systems is discussed
