What I use:
- SD card with coreelec and ssh enable
- emmc installed on C4
- method 3 from https://forum.odroid.com/viewtopic.php?t=37425&p=278010 with replacement of image link for C4
If emmc is empty or for another device (mine was with ubuntu for xu4) C4 will boot from SD.
I clean emmc before writing CE but I do not remember the procedure.
A better guide here: Copy core elec from usb to sdcard