Prickly pear is problematic for several reasons, but the way it reproduces makes it difficult to control because cutting it can make it spread. This means that each pad has the ability to root and form a new colony if it is detached from the “mother plant.” If you all mow that 4 acres to normally control weeds and stimulate grass growth - it's spreading the cactus. This cactus grows in open, dry areas, often on calcareous rock or thin soils. The University of Florida has some research on herbicides that are effective and so has Texas AMU. https://edis.ifas.ufl.edu/ag363
https://texnat.tamu.edu/about/brush-busters/pricklypear/  I hope this information is helpful.