Technically we have a fourth dot, its called Shadowfiend/Mindbender. Even if you don't consider this a real dot, if they were to give us a fourth dot, they would just nerf our other 3 to compensate.
What Shadow lacks right now is high sustained damage during heavy movement encounters. If Lei Shen did not have ball lightnings for us to halo/mind sear, we would be absolute garbage.
A buff to MB or MS would be the only real solution for increasing damage during heavy movement fights, Shadow has always been competitive on multi-target fights (Not including lolchainlightning) but this leaves us to be average for single target, and below average with heavy movement. That 4pc tier for next patch looks alright, but I find it lackluster.
We already received a new dot in 5.2, Insanity, and it fixed our single target issues, but it also created a new one. If you've, for example, done any Challenge modes, where you can hardly get over 10k haste, you'd realize how it made us even more haste dependent, and just how Insanity is a useless spell at low haste. Adding even another dot would just do the same.
What we need is an increase in value of other stats, namely crit and mastery, in some way. Either making VT Apparitions baseline, and make it spawn them on crits, same way SW: P does, or adding some other mechanic that makes those stats worth more than nothing. I think that's what they're trying to do with our new set bonuses, but once we get rid of that set the issues will be back again. Hopefully we get an overhaul by then tho.
When you cried I'd wipe away all of your tears
When you'd scream I'd fight away all of your fears
And I held your hand through all of these years
Priests with Dragonwrath always did good numbers.
The time of priests have ended... Time for the Warlocks has come!