Reloading Samba4's smb.conf without restarting the service

Try to use this: smbcontrol all reload-config

You can try with sending SIGHUP signal to smbd process

killall -HUP smbd nmbd

NOTE: Be careful using killall on Unix. Running killall on Solaris on root would send kill signals to all processes! [en.wikipedia.org/wiki/Killall]