The hexagonal uranium compound UNi 4B has been shown to exhibit strongly anisotropic hybridization of f- and d-electron states. We performed low-field (3 mT) and high-field (40 T) magnetization measurements from 1.4 K to 30 K on a single crystal of this antiferrromagnetically ordering compound, T N=20 K, to further investigate the magnetic anisotropy. A phase diagram, including a spin-flop transition around 9 T, is presented. The observed large values of the electronic specific heat at low temperatures are attributed to in-plane magnetic fluctuations, which persist far below T N. No superconductivity was found down to 40 mK. The relevance of strong 5 f-3d hybridization is confirmed by extensive experiments on a series of diluted compounds UCo xNi 4-xB(0≤×≤4).