The aircraft departed from London Gatwick Airport for a scheduled flight to Las Vegas. Following retraction of the landing gear after takeoff, low quantity and pressure warnings occurred on hydraulic system 4, due to a hydraulic fluid leak. The required checklists were completed and the aircraft returned to land at London Gatwick Airport. As the landing gear extended during the approach, the right wing landing gear struck the gear door, preventing the gear leg from fully deploying. The crew carried out a go-around and, following a period of troubleshooting and associated preparation, a non-normal landing was successfully completed. It was subsequently determined that the hydraulic retract actuator on the right wing landing gear had been incorrectly installed. Four Safety Recommendations have been made.