We report the first photoluminescence (PL) measurements on stacked layers of self-assembled InP quantum dots in pulsed magnetic fields up to 50 T at 4.2 K. A strong increase of the diamagnetic shift with magnetic field parallel and perpendicular to the growth direction is observed by decreasing the stacked layer separation from 8 to 4 nm, indicating that in latter case the quantum dot layers are strongly coupled. This is confirmed by the decrease in PL line width, which is determined by the height homogeneity of a single dot or by the fluctuation of the total height of the coupled layers.