Just picked up my 4 set and I figured I would share some information I learned about it so far:
The Golden Apparition is instant. In fact, when you Penance, the Golden Apparition healing will be before any Penance ticks get off.
With the above statement in mind, Golden Apparition procs are based on when Penance CD is burned. Penance can do 0 healing (immediately interrupting the cast) and a Golden Apparition can still proc. This means it does not proc based on each Penance tick.
Penance healing or damage can proc the Apparition.
The Golden Apparition functions with Disc mastery. It can crit and will create a Divine Aegis bubble on its target. I have not tested the 4set with Holy, but I am assuming that it also functions with Holy mastery.
The healing done by Golden Apparition scales with healing modifiers, including Disc mastery, Archangel, Jin'rokh pools, etc. Its base healing is 100k, and is not effected by intellect or spellpower (based on the tooltip).
I hope this helps! I'm personally always asking questions about new tier bonuses towards the start of tiers, so I figured I'd share some insight, whether or not it's considered pretty obvious. Let me know if there's something you want me to test out. I'll post logs later after doing some raids.
Don't know if it's been changed, but on PTR it wasn't quite instant. It had a travel time, but it moved very quickly (much faster than Penance does) and would reach its target in <1s even at 40 yards. Edit: talking about the Golden Apparition
Bear in mind that's for a holy with the reduced CoH cd, will no doubt be statistically worse for a disc with a longer penance cd.
And if you compare this 4set with the previous tiers reduced penance cd, I'd say it's a bit shabby after all.
Though of course you are right that it's better than nothing, who wouldn't like a free smart heal. Not sure why I expected something a bit more exciting.
They might have normalized the proc change for penance and CoH, if they did then it might be better or worse for disc depending on how/if they took ToT into account. Also, keep in mind that penance is more versatile than CoH being that it can be used as single target or multi target (3) heal. The only time using penance on CD isn't the best you can do is if you are in the process of stacking Spirit Shell or if there nothing happening at all (no mobs and no damage).
You say that as if a holy priest wouldnt use CoH on cd just for the kicks of using it.
I dare to say very many holy's do, especially with the 4set. I know ours do, and so do most I've played with outside the guild as well. Then again, why the hell not unless something massive is going to happen within that small cd gap.
I wouldn't say that arguing penance > CoH makes the 4set any more viable, either way.
Looking at disc 4set logs the healing seems to account for about 2,5% of overall healing most fights (heroic 25).