Catalyzes the hydroxylation of 4-hydroxyphenylacetic acid (4HPA), leading to the production of 3,4-dihydroxyphenylacetic acid (DHPA). 4-hydroxyphenylacetate + FADH2 + O2 = 3,4-dihydroxyphenylacetate + FAD + H(+) + H2O Aromatic compound metabolism; 4-hydroxyphenylacetate degradation; pyruvate and succinate semialdehyde from 4-hydroxyphenylacetate: step 1/7. Belongs to the FADH(2)-utilizing monooxygenase family.