B747 Captain reported a repetitive main system hydraulic overheat.

Narrative

Hydraulic system 4 overheated on approach to ZZZ. Executed go-around and ran QRH procedure. Landed with system depressurized.This was the second leg this airplane flew with the same hydraulic system 4 overheat event. In ZZZ1; I discussed the in-bound write up with maintenance control and contract line maintenance. Maintenance control seemed to indicate a cooling valve was bad and needed to be replaced. In speaking with line maintenance; they could not find anything wrong with the system by simply reviewing maintenance message in the maintenance computer). I requested they run the engine and test the hydraulic system prior to me accepting the airplane. They did and I observed system four operating without an EICAS message; but definitely hotter than the other 3 hydraulic systems.I told them I would accept the plane; but if I saw a temperature spike prior to takeoff; I would be coming back to the blocks. We took off uneventfully and the system operated without an EICAS message (but; again; higher than the other 3 systems) during the entire flight until we started extending flaps on approach to ZZZ. Somewhere around 2000 feet on approach; the overheat advisory message appeared. We executed a go-around; ran the QRH procedure; and landed in ZZZ on the longest runway with hydraulic system 4 depressurized (alternate landing gear extension was used).After landing; I briefed the line maintenance person in ZZZ of what happened. I explained that the airplane now flew two legs with the same hydraulic system overheating. (I also put 'repeat discrepancy' in the logbook)This airplane then attempted to fly to ZZZ2 a few hours after we blocked in and experience the same hydraulic system 4 overheat for the THIRD consecutive leg. It was still not fixed. During that flight; the crew dumped fuel; requested priority handling; and returned to ZZZ.Cause: Maintenance didn't take any action in ZZZ1 after the first write up on the system. I.E. They looked at the computer; but did not fix or change any part. Our line maintenance procedures don't emphasize troubleshooting what the problem is; beyond what the airplane reports.If a major issue cannot be corrected after two legs of flying; the plane should not fly again until the issue is corrected. It is obvious our line maintenance procedures are not adequate to prevent these types of repeatable issues with the airplanes.I don't know if the 3rd crew was briefed on the history of the airplane. But our maintenance briefings do NOT include a discussion of current issues with the airplane that have been repeating. The briefing is not too useful for the crewmembers and should be improved.Suggestions: Improve our line maintenance procedures and quality.