We report a study into the field-induced magnetisation of the two frustrated rare earth tetraborides, HoB₄ and NdB₄. NdB₄ shows a fractional magnetisation plateau occurring at M/M_sat ≈ 1/5 before saturating in a field of 33 kOe. On cooling down to 0.5 K the temperature dependent susceptibility of NdB₄ shows an unconventional transition where the system returns to the zero field antiferromagnetic state from a higher-temperature ferrimagnetic state. We are able to reconstruct the magnetic phase diagram of NdB₄ from the magnetisation, susceptibility and resistivity measurements for both H//c and H⊥c. For HoB₄, the most interesting behaviour is found at the lowest temperature of 0.5 K, where the field dependent magnetisation demonstrates a new fractional 1/2-magnetisation plateau. Further insight into the relations between the exchange interactions and single ion effects is gained through high-field magnetisation measurements in both HoB₄ and NdB₄.