round robin is where:

1st press: Char A casts Spell A
2nd press: Char B casts Spell A
3rd press: Char C casts Spell A

etc.

You seem to want something different in which case you don't use round robin at all just:

Step 1

Char A casts Spell A
Char B casts Spell A
Char C casts Spell A

etc. do not advance 4.0 sec


Step 2

Char A casts Spell B
Char B casts Spell B
Char C casts Spell B

etc.

If you want to you can set up 5 steps and have the characters cast different spells each of the 5 steps giving you a "5 steped round robin" or whatever you would call it.

Step 2