let the actual depth of water in container is dactual
then apparent depth of water in container is dapparent
dapparent = dactual−4
the relation between refractive index and depth is
refractiveindex=actualdepthapparentdepth
μ=dactualdapparent
43=dactualdactual−4⇒dactual=16cm
Hence the actual depth of water is 16 cm.