How did you populate the TFTP directory?
In general copying all files from the FAT partition of the SD card should just work (after adjusting cmdline.txt). If you populated it from the contents of the tarball you'll need to rename KERNEL to kernel.img.
Netbooting LE10 on RPi4s is working fine here, I've been doing that for a while now for easy development testing.
I chose to organize TFTP and NFS directories using the RPi4 serial number at the top level, RPi4 firmware looks there first when TFTP-booting (and then falls back to TFTP root dir). This makes it easy to serve different (LE) versions to different RPis and the LE init script also supports that quite well, it substitutes "@UID@" in boot/disk with the serial - so I can use an identical cmdline.txt for all installations. The relevant part of cmdline.txt looks like this here:
ip=dhcp boot=NFS=192.168.1.20:/wd/srv/tftp/@UID@ disk=NFS=192.168.1.20:/wd/srv/le-storage/@UID@
TFTP root directory (I'm using atftpd here) is /wd/srv/tftp and /wd/srv is exported via NFS, and my DHCP server announces the TFTP server via option 66 (tftp in dnsmasq) in DHCP replies.
Check your tftp logs, also for missing files. For reference here's the tftp log output from a boot here:
atftpd[2270]: Serving 8eb25f41/start4.elf to 192.168.1.70:7088
atftpd[2270]: File /wd/srv/tftp/8eb25f41/start4.elf not found
atftpd[2270]: Serving 8eb25f41/start.elf to 192.168.1.70:7089
atftpd[2270]: Serving 8eb25f41/config.txt to 192.168.1.70:7090
atftpd[2270]: Serving 8eb25f41/vl805.sig to 192.168.1.70:7091
atftpd[2270]: File /wd/srv/tftp/8eb25f41/vl805.sig not found
atftpd[2270]: Serving 8eb25f41/pieeprom.sig to 192.168.1.70:7092
atftpd[2270]: File /wd/srv/tftp/8eb25f41/pieeprom.sig not found
atftpd[2270]: Serving 8eb25f41/recover4.elf to 192.168.1.70:7093
atftpd[2270]: File /wd/srv/tftp/8eb25f41/recover4.elf not found
atftpd[2270]: Serving 8eb25f41/recovery.elf to 192.168.1.70:7094
atftpd[2270]: File /wd/srv/tftp/8eb25f41/recovery.elf not found
atftpd[2270]: Serving 8eb25f41/start4.elf to 192.168.1.70:7095
atftpd[2270]: File /wd/srv/tftp/8eb25f41/start4.elf not found
atftpd[2270]: Serving 8eb25f41/start.elf to 192.168.1.70:7096
atftpd[2270]: Serving 8eb25f41/fixup.dat to 192.168.1.70:7097
atftpd[2270]: Serving 8eb25f41/recovery.elf to 192.168.1.70:49153
atftpd[2270]: File /wd/srv/tftp/8eb25f41/recovery.elf not found
atftpd[2270]: Serving 8eb25f41/config.txt to 192.168.1.70:49154
atftpd[2270]: Serving 8eb25f41/config.txt to 192.168.1.70:49155
atftpd[2270]: Serving 8eb25f41/distroconfig.txt to 192.168.1.70:49156
atftpd[2270]: Serving 8eb25f41/distroconfig.txt to 192.168.1.70:49157
atftpd[2270]: Serving 8eb25f41/dt-blob.bin to 192.168.1.70:49158
atftpd[2270]: File /wd/srv/tftp/8eb25f41/dt-blob.bin not found
atftpd[2270]: Serving 8eb25f41/recovery.elf to 192.168.1.70:49159
atftpd[2270]: File /wd/srv/tftp/8eb25f41/recovery.elf not found
atftpd[2270]: Serving 8eb25f41/config.txt to 192.168.1.70:49160
atftpd[2270]: Serving 8eb25f41/config.txt to 192.168.1.70:49161
atftpd[2270]: Serving 8eb25f41/distroconfig.txt to 192.168.1.70:49162
atftpd[2270]: Serving 8eb25f41/distroconfig.txt to 192.168.1.70:49163
atftpd[2270]: Serving 8eb25f41/bootcfg.txt to 192.168.1.70:49164
atftpd[2270]: File /wd/srv/tftp/8eb25f41/bootcfg.txt not found
atftpd[2270]: Serving 8eb25f41/edid.cpio to 192.168.1.70:49165
atftpd[2270]: Serving 8eb25f41/edid.cpio to 192.168.1.70:49166
atftpd[2270]: Serving 8eb25f41/bcm2711-rpi-4-b.dtb to 192.168.1.70:49167
atftpd[2270]: Serving 8eb25f41/bcm2711-rpi-4-b.dtb to 192.168.1.70:49168
atftpd[2270]: Serving 8eb25f41/overlays/overlay_map.dtb to 192.168.1.70:49169
atftpd[2270]: Serving 8eb25f41/overlays/overlay_map.dtb to 192.168.1.70:49170
atftpd[2270]: Serving 8eb25f41/config.txt to 192.168.1.70:49171
atftpd[2270]: Serving 8eb25f41/config.txt to 192.168.1.70:49172
atftpd[2270]: Serving 8eb25f41/distroconfig.txt to 192.168.1.70:49173
atftpd[2270]: Serving 8eb25f41/distroconfig.txt to 192.168.1.70:49174
atftpd[2270]: Serving 8eb25f41/overlays/vc4-kms-v3d-pi4.dtbo to 192.168.1.70:49175
atftpd[2270]: Serving 8eb25f41/overlays/vc4-kms-v3d-pi4.dtbo to 192.168.1.70:49176
atftpd[2270]: Serving 8eb25f41/overlays/rpivid-v4l2.dtbo to 192.168.1.70:49177
atftpd[2270]: Serving 8eb25f41/overlays/rpivid-v4l2.dtbo to 192.168.1.70:49178
atftpd[2270]: Serving 8eb25f41/overlays/disable-bt.dtbo to 192.168.1.70:49179
atftpd[2270]: Serving 8eb25f41/overlays/disable-bt.dtbo to 192.168.1.70:49180
atftpd[2270]: Serving 8eb25f41/overlays/disable-wifi.dtbo to 192.168.1.70:49181
atftpd[2270]: Serving 8eb25f41/overlays/disable-wifi.dtbo to 192.168.1.70:49182
atftpd[2270]: Serving 8eb25f41/overlays/gpio-ir.dtbo to 192.168.1.70:49183
atftpd[2270]: Serving 8eb25f41/overlays/gpio-ir.dtbo to 192.168.1.70:49184
atftpd[2270]: Serving 8eb25f41/cmdline.txt to 192.168.1.70:49185
atftpd[2270]: Serving 8eb25f41/cmdline.txt to 192.168.1.70:49186
atftpd[2270]: Serving 8eb25f41/recovery8.img to 192.168.1.70:49187
atftpd[2270]: File /wd/srv/tftp/8eb25f41/recovery8.img not found
atftpd[2270]: Serving 8eb25f41/recovery8-32.img to 192.168.1.70:49188
atftpd[2270]: File /wd/srv/tftp/8eb25f41/recovery8-32.img not found
atftpd[2270]: Serving 8eb25f41/recovery7l.img to 192.168.1.70:49189
atftpd[2270]: File /wd/srv/tftp/8eb25f41/recovery7l.img not found
atftpd[2270]: Serving 8eb25f41/recovery7.img to 192.168.1.70:49190
atftpd[2270]: File /wd/srv/tftp/8eb25f41/recovery7.img not found
atftpd[2270]: Serving 8eb25f41/recovery.img to 192.168.1.70:49191
atftpd[2270]: File /wd/srv/tftp/8eb25f41/recovery.img not found
atftpd[2270]: Serving 8eb25f41/kernel8.img to 192.168.1.70:49192
atftpd[2270]: File /wd/srv/tftp/8eb25f41/kernel8.img not found
atftpd[2270]: Serving 8eb25f41/kernel8-32.img to 192.168.1.70:49193
atftpd[2270]: File /wd/srv/tftp/8eb25f41/kernel8-32.img not found
atftpd[2270]: Serving 8eb25f41/kernel7l.img to 192.168.1.70:49194
atftpd[2270]: File /wd/srv/tftp/8eb25f41/kernel7l.img not found
atftpd[2270]: Serving 8eb25f41/kernel7.img to 192.168.1.70:49195
atftpd[2270]: File /wd/srv/tftp/8eb25f41/kernel7.img not found
atftpd[2270]: Serving 8eb25f41/kernel.img to 192.168.1.70:49196
atftpd[2270]: Serving 8eb25f41/kernel.img to 192.168.1.70:49197
atftpd[2270]: Serving 8eb25f41/armstub8-32-gic.bin to 192.168.1.70:49198
atftpd[2270]: File /wd/srv/tftp/8eb25f41/armstub8-32-gic.bin not found
atftpd[2270]: Serving 8eb25f41/kernel.img to 192.168.1.70:49199
atftpd[2270]: Serving 8eb25f41/kernel.img to 192.168.1.70:49200
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so long,
Hias