In the prior version of spell system you could click on a spell and press unquip to clear the slot. For my PVP loadouts sometimes I only want 2 or 3 spells to fill the 2/3 up faster than with 3/4s spells. I accidently added a spell to the 4th slot and not it seems stuck.
You can’t, but also you fill up spells faster if you run 4, then if you run less because of the bonus. Just run a different color or put the spell you want to use the least in the back and you’re good to go.