The S-IVB also ongoing to thrust in a lower level by venting gaseous hydrogen, to help keep propellants settled of their tanks and stop gaseous cavities from forming in propellant feed strains. This venting also taken care of Protected pressures as liquid hydrogen boiled off in the gas tank. https://lowpowersemiconductors65296.kylieblog.com/34471324/apollo4-blue-plus-an-overview