Hello, i was trying to netboot my rpi4 and the solution to do this is to add these lines to your dhcp server config (i'm using dnsmasq):
dhcp-match=set:client_is_a_pi,97,0:52:50:69:34
dhcp-match=set:client_is_a_pi,97,0:34:69:50:52
pxe-service=tag:client_is_a_pi,0,"Raspberry Pi Boot"
rpi4 dhcp-match client contains special bytes in GUID (97). The first one is for RPI4 (0:52:50).