Answer:
phosphorous acid
Step-by-step explanation:
PO4 (phsophate) is a polyatomic, so when the subscript changes, so does the name and it becomes phosphite (one less oxygen atom)
if the name of the polyatomic ends in ite, the ending of the acid name becomes ous making it phosphorous acid.
You don't need to worry about the hydrogen in the naming process because it is an oxyacid, where you only really worry about the polyatomic (acid tells you there is hydrogen)