4-hydroxytryptamine kinase; part of the gene cluster that mediates the biosynthesis of psilocybin, a psychotropic tryptamine-derived natural product (PubMed:28763571, PubMed:30283667). The first step in the pathway is the decarboxylation of L-tryptophan to tryptamine by the decarboxylase psiD. PsiD does not decarboxylate phenylalanine, tyrosine, or 5-hydroxy- L -tryptophan (5-HTP) (PubMed:30283667). 4-hydroxy-L-tryptophan is accepted as substrate by psiD as well. The cytochrome P450 monooxygenase psiH then converts tryptamine to 4-hydroxytryptamine. The kinase psiK catalyzes the 4-O-phosphorylation step by converting 4-hydroxytryptamine into norbaeocystin. The methyltransferase psiM then catalyzes iterative methyl transfer to the amino group of norbaeocystin to yield psilocybin via a monomethylated intermediate, baeocystin. PsiK kinase can also turn psilocin into psilocybin. This activity may represent a protective mechanism to rephosphorylate the unstable psilocin to the stable psilocybin in case of intracellular ester cleavage (By similarity). 4-hydroxytryptamine + ATP = 4-hydoxytryptamine 4-phosphate + ADP + H(+) Secondary metabolite biosynthesis. Belongs to the methylthioribose kinase family.