I have a USB 4 into 1 external extra item, would it be safe to plug this into the USB, not to use more than One Device, but keep them all Charged and memory intact

I cannot see an issue, the car will still supply it's maximum rating of 1A or 2A charge (whatever it supplies exactly I don't know)

 

It will just mean everything has to share the juice available.

 

Only exception would be if your 4 port USB hub is electrically flakey.

The power from the factory usb port is quite low. If it were me I'd connect what I needed to the car (music storage?) Then the other stuff to a 12v usb port either with a dedicated unit like this or the hub you describe.