Why does the manual for the inner tube say the maximum pressure is 4.5 PSI?
Most likely this low number refers to inflating the tubes without the tire, as a balloon.
When installed inside a tire, the majority of air pressure is held by the tire casing, not by the tube.
A similar warning about not inflating outside of/without a tire came with my Tubolito inner tubes, but the wording was more clear in my case.