Regularity for Hamiltonian stationary equations in $\mathbb{R}_{n\leq 4}^{n}$

In this paper, we study the regularity of solutions to the Hamiltonian stationary equation in complex Euclidean space. We show that in dimensions $n\leq 4$, for all values of the Lagrangian phase, any $C^{1,1}$ solution is smooth and derive a $C^{k,\alpha}$ estimate for it, where $k \geq 2$.