When you use the commands
/effect @a minecraft:instant_health 1 0
and
/effect @a minecraft:instant_damage 1 0
in one gametick (with commandblocks), the instant_damage command will always be executet first, no matter in which order you put the command blocks. You can see that in gamemode 0/2 when you are at 6 HP or less, you will die instantly in stead of first get healed for 4 HP and then get damaged for 6 HP.