Sums of Four and More Unit Fractions and Approximate Parametrizations

We prove new upper bounds on the number of representations of rational numbers mn as a sum of four unit fractions, giving five different regions, depending on the size of m in terms of n. In particular, we improve the most relevant cases, when m is small, and when m is close to n. The improvements stem from not only studying complete parametrizations of the set of solutions, but simplifying this set appropriately. Certain subsets of all parameters define the set of all solutions, up to applications of divisor functions, which has little impact on the upper bound of the number of solutions. These 'approximate parametrizations' were the key point to enable computer programmes to filter through a large number of equations and inequalities. Furthermore, this result leads to new upper bounds for the number of representations of rational numbers as sums of more than four unit fractions.