GdTb-FeCo based quaternary system of various thicknesses (30, 50, 75, 150, and 300 nm) is explored in pursuit of tuneable perpendicular magnetic anisotropy (PMA). Spin reorientation and strong PMA is evident at higher film thickness. A variety of microscopic domains can be referred to as disorder in domains, have been observed. Gd-like and Tb-like contributing domains are observed in various films. A critical thickness is observed at 150 nm, where most of the Tb-sublattices are dominated over the Gd-sublattices. Magnetic parameters do not follow the trend at 150 nm, which could be attributed to a lesser extent of pinning sites impeded to domain wall motions. The experimental finding is complemented with 3D micromagnetic simulations. © 2021 IOP Publishing Ltd.