FreeBSDONLYrpi4netbootIMG

You can boot this image from uSD as well as from an USB medium on the RPI4.

It will straight netboot up your RPI4 (without any necessary user-interaction or configuration while booting!), presumed that you have configured your pxe-filesystem-server
like described in the the FreeBSD-handbook :
https://docs.freebsd.org/en/books/handbook/advanced-networking/#network-diskless
You DON`T need to place u-boot or firmware to your pxe-server .
download-link :
https://sourceforge.net/projects/freebsdonlyrpi3and4netbootimg/files/sec_freebsd_rpi_netboot_usd_usb_klaus.img.xz/download
(download file size : 84,1MB), decompressed .img-size :716,4MB, .
usage :
/*decompress:
'xz -d freebsd_rpi_netboot_usd_usb_klaus.img.xz' */
/* copy (example supposed your device is /dev/ridsk3 ) :
'dd if=freebsd_rpi_netboot_usd_usb_klaus.img of=/dev/rdisk3 bs=4m' */
if your boot hangs after the following output :
net0: cannot set rx. filters (status=3)/
that means wrong pxe-server-configuration.