Also, in strict 10 man, because of the lower item level and therefore less haste available, the cost of taking 4T10 is higher in terms of spell power and/or spec.

That seems correct--wowhead lists a 10% proc rate for all trinkets in that family (Memento of Tyrande, Sif's Remembrance, Show of Faith, Purified Lunar Dust). 55s CD is probably the best model.

Shall we start a t10 4set question/test list for next week?

First of all: Does it consume the Rejuv? If so, report to Blizz asap!
Does the created Rejuv tick for the same amount? Is it affected by Glyph of Rapid Rejuv?
How far can it jump?
How does it choose the target to jump to? Does it prioritize people without your Rejuv over lowest health % or deficit?
What happens if it procs and everyone in range of the jump has a Rejuv from you on them? Does it refresh the Rejuv with the shortest duration?
What happens if it procs and no one is in range of the jump? Does it refresh the Rejuv it proc'd from?

What else?

Alright, how's this?

Who 'casts' the proc Rejuv, the caster of the original Rejuv, or the target the original Rejuv was on?
Can it proc if you're dead, or stunned/silenced/unable to cast?
Is the range centered around the you, or around the target the original Rejuv was ticking on?
Can the new Rejuv proc another new Rejuv?
Does the proc Rejuv stack with your Rejuv on a target or replace an existing Rejuv?
Does the proc put you in combat if the target was in combat? Does it cause Threat?

Is the duration of the new rejuv affected by Glyph of Rapid Rejuv?
Is the period of the new rejuv affected by Glyph of Rapid Rejuv?
Can the new Rejuv proc Glyph of Rejuvenation?

Will it only jump to party/raid members, or any friendly target in range? Pets? Vehicles? Totems?
Will it jump to friendly NPCs? Neutral NPCs?
Will it jump to PVP Flagged players, when the caster is not flagged? Would doing so flag the caster? Does it count as a pvp action and aggro guards?

Will procs during Sindragosa's Unchained Magic result in Instability?

I suggest a few basic tests if someone gets it today. You need 2 willing participants, one of which should be a Warlock.

1. Keeping Rejuv active on all 3 of you until you get the 4t10 proc. Will it overwrite an existing Rejuv or stack with it? If it overwrites one, will it always overwrite the one with the lowest duration or a random one?
2. Keeping Rejuv active on all 3 of you while the Warlock uses Life Tap so that he is never at full health. Will the proc always target the Warlock or anyone of you?
3. Keeping Rejuv active on 2 of you. Will the proc always target the third (who doesn't have a Rejuv on him) or is the target random?
4. Keeping Rejuv active on yourself and the Warlock who uses LT to keep himself at low health. Will the proc target the full HP group member without Rejuv on him or the low HP Warlock that already has a Rejuv running on him? Or is it completely random?

All of those would have to be done multiple times to get some statistics. I think those tests cover the most interesting things about the proc and I'd be extremely happy if someone could find the time to do them

I'll be doing a bunch of testing tomorrow. From what I can see, the thing that will really make or break the 4 set bonus in terms of usability will be if it can proc off itself. Hopefully, it will spawn another spell cast event so it doesn't cause any UI problems too.

I don't agree. Gettnig a secondary proc is only a minor effect, with 4t10 in the order of magnitude of 10% for Rejuv the secondary effect will only give around 1%, not nearly enough to be significant.

What's really important is how the proc targeting works. Will it stack with existing Rejuvs or will it overwrite them? Will it target low HP raid members (no matter if they already have Rejuv on them, most probably wasting the proc in case it overwrites an existing one) or will it only target people without Rejuv (greatly helping out on raid blanketing)?
That's why I think the 4 simple tests I posted above are the most essential (and luckily they can be done quick and easy as soon as anyone got the set, since I missed raiding and dailies over Christmas for me this will sadly take one more week so I can't do them myself yet).

I fear it will work like WG and just target the lowest HP raid member in range, overwriting any existing effect on it. For WG this is not an issue since only the last tick can still be active on the target (6s cooldown, 7s duration) but for Rejuv it would almost render the set bonus useless.

I only did about 30 minutes of testing, so my sample size is not large by any means.

Group was three people including a lock with a pet.

1. It can jump on application, not just ticks.
2. I was unable to get it to jump when all targets had rejuv.
3. When all players had rejuv it jumped to the pet (no rejuv) 4/4 times.
3a. Repeating with lock lifetapping it still jumped to pet (no rejuv) 2/2 times.
4. With one rejuv up, it jumped to the lifetapping warlock 5/5 times.

I didn't think to check the range of the jump.

That sounds to me like: smart targeted, doesn't overwrite existing HoT's, and as a bonus, 7 chances to proc instead of 6? That's about as good as we could have hoped for. More confirmation would be nice.

When you say "jump". Was this a new one created or the rejuv just moved?

Also, curious to know if the ticks were the same power.

I got on my roommates druid to test this as well, as he received his 4PC. I would agree to everything, and like to add that I am almost positive that it does not proc of Glyph of Rejuvenation, as the heal is called Glyph of Rejuve, not Rejuvenation itself. I deleted the combat logs like an idiot, but I had obtained ~ 450 procs of Glyph of Rejuve with the 4PC on, and no evidence suggests it proc'd off it.

[x] Smart Heal
[x] Application of Rejuve
[x] No overriding (good for swiftmend)
[ ] Glyph of Rejuvenation (?)

It did seem to jump about 15 yards, but I did not test that. Most likely 30 yards? Maybe only 12.