The Raspberry Pi 400 doesn't have a normal Raspberry Pi inside the keyboard unit. It's, in fact, a special PCB which is essentially a Raspberry Pi 4 (4GB) in a different shape, size and with a slightly different set of connectors (see picture below from our teardown article)
There's no way to fit your own Raspberry Pi inside the keyboard (for those of you hoping to install an 8GB board!) so just the standard 4GB option is available currently.
We also have another support article on an 8GB version (there isn't one, yet) which may help answer that side of the question too.