"The refractive index of water is 4 / 3" means the refractive
index of water with respect to air. "The refractive index of glass
is 3 /2" means the refractive index of glass with respect to
air.
You change the question as ," what is the refractive index of
glass with respect to water?
The answer is it is the ratio of refractive index of of glass
with respect to air to the refractive index of water with respect
to air =( 3/2) divided by( 4/3) = 1.125