The RPi4 8 GB model is not supported by the current port version: Memory isn't recognized as 8 GB, and the bcm_xhci firmware does not load, disabling USB. Both is supported with the 20201201 firmware according to my tests. Please update the port. Thanks!
A commit references this bug: Author: manu Date: Sun Dec 13 11:53:58 UTC 2020 New revision: 557893 URL: https://svnweb.freebsd.org/changeset/ports/557893 Log: sysutils/rpi-firmware: Update to 20201201 Update to the latest firmware released. This should fix booting on the 8GB model and this also add dtb for the 400 and CM4 variant. PR: 251801 Changes: head/sysutils/rpi-firmware/Makefile head/sysutils/rpi-firmware/distinfo head/sysutils/rpi-firmware/pkg-plist
Should be good now, please reopen if that doesn't fix the issue for you.