It depends on what you're willing to do, if you want to get your burst done straight away, start of by casting Mind Flay twice in order to get 5 stacks of Evangelism. I would suggest making a macro, which could enable a volcanic potion, trinket procs, and follow it up by /cast Mind Spike, then release your Shadow Fiend, you will then be able to cast mind spike straight after as your Shadow fiend will leap straight to the target and start pew pew'ing. As soon as you know that your Shadow Fiend is over (15 seconds) make sure that you finish it off with a Mind Blast so your dots don't disappear due to the Mind Spike debuff.
You should be able to do your rotation atleast twice, unless you've got enough haste, not sure if its possible to cast Mind Blast x3 in the rotation.
After that, you can start DPS'ing and just rince and repeat.
You want to put in "/cancelaura mind melt" after /cast mind spike and before /cast mind blast. This way you will do a regular mind blast cast which will enable your shadowfiend to get an additional hit in before your mind blast cast is finished. Some players choose to simply delay the instant cast by 1 sec and some choose to cast mind flay for 1-2 ticks, but putting in /cancelaura mind melt is the best option seeing as you activate GCD as fast as possible and your mind blast will still hit just after gaining 3 orbs.
Hijacking a bit =X I don't use /cancelaura Mind Melt and if I'm MBing next, I wait that part of a second to MB (or fill it with a partial MF). My question is with the Legendary Staff though. If you immediately cast MB after an MS, don't you run the risk of having 0 orbs due to the staff doubling up an MS?
And after you have MS at 3 stacks and MB on CD (waiting for it to come back up), do you continue to use MS to use the orbs until you're able to MB? Or would MF to build Evangelism be more ideal? Also, do you ramp up any MS before popping SF + AA (I always pop SF + AA, then start MSx3 -> MB. Is it better to build MS before popping SF?)
I honestly haven't looked into the proper rotation for 4set, so if someone can point me in the right direction that'd be great. I sort of just do my own thing when I do the 4set rotation. Also, are you always supposed to pair AA and SF when doing this rotation? Is this rotation still worth it if you don't have AA up?
There are different rotations that give different results and such.
Personally I use #2 because it will have me at 5 stacks of evangelism at the end of MS/MB rotation and I don't need to use a cancel aura macro.