4 piece: Increases the damage done by your Starfire and Wrath spells by 4%.
At first glance I am happy. Its a new set bonus that is not an inferior copy of a previous set bonus. Plus, this set bonus is a buff when compared to the 3% increase to crit chance. However, there are a few important things to remember.
While the change above is nice, the best news from this PTR build is that 2T8 hasn't been nerfed yet. As Hamlet, and several other posters have pointed out, the 4T9 set bonus really doesn't matter while 2T8 remains in its current form. The 2T8 set bonus is so good with the New Eclipse, that Blizzard would have a very hard time creating a 4T9 set bonus would compare without escalating our expectations for future set bonuses. So, Blizzard has to break the cycle. Hopefully they will do that by not considering 2T8 when they create future set bonuses, but leave it as is, but the other option is to nerf it. I think I speak for almost all moonkin when I say I hope that doesn't happen.
Another important thing to remember is that currently we don't know if this is an additive buff or a Multiplicative buff. If it fits the normal pattern then this is an additive buff because it affects specific spells. Therefore a 4% buff is not really a 4% buff. In reality it will be a little less for a couple of reasons. The obvious reason is that we cast more spells than Wrath and Starfire, and those other spells are unaffected by the buff. Also, since we already have other additive buffs like Moonfury and Solar Eclipse then the impact of 4T9 will be reduced when they are combined.
As you might have guessed, I ran these changes through my model a couple of different ways to compare 2T8 and 4T9. I ran my model without either buff first to come up with a good baseline. Then I ran it once with 2T8, once with 4T9, and then once with 4T9 and some added stats from the gear up grade. Here are the results.
Adding the 2T8 set bonus increased my model DPS by 6.1%. That is huge if you didn't know and it is obvious right off the bat that 4T9 cannot compete with it buy itself. Adding the 4T9 set bonuses into the model showed that increased my model DPS by 2.5%. However, 4T9 does not come by itself.
Moving to 4T9 will upgrade your stats as well as change you set bonus. Since the difference between 2T8 and 4T9 is so great, I jumped right to T9.258 when adding the additional stats to the model. I also assumed that the two pieces of T8 we would keep are the Legs and the Shoulders. I chose these two items because the T9 Helm and Chest seem to be the best, and I wanted to limit the amount of Hit rating I got from the T8 set pieces. When I compared the legs and shoulders between the two sets I saw that T9.258 had the equivalent of 132 additional Spell Power, the equivalent of 29 additional Crit Rating, and 43 additional Hit rating. To be conservative I increased the Spell Power amount on my model 153 assuming that half of the additional hit rating would be converted into SP some where, and increased my crit chance by 0.64%. The additional stats combined with the 4T9 set bonus this increased my model DPS by 6.9%
Some of you may be surprised by this result. A lot of the rhetoric I've seen passed around on the internet seemed to assume that 2T8 was unbeatable. Well, it is not, and we all new that some level of gear would make it irrelevant. I just don't think anyone expected that T9 would be that gear level.
However, lets try and look at the bright side. This is a great argument for Blizzard to not nerf 2T8.
So let me try and summarize these results.
The new 4T9 is an improvement over the previous versions, but 2T8 is so good that Blizzard would have a hard time come up with a reasonable set bonus to compete with it. When you compare 2T8 to 4T9 directly, the 2T8 set bonus is clearly better by a wide margin.
However, if you upgrade to 4T9 your picking up more then a new set bonus. The additional stats you will pick up along with 4T9 will close the gap, and according to my model eliminate the gap if your using the highest iLevel of T9.
It is important to note how close the two numbers are. This shows that there is very little difference between using 2T8.226 and 4T9.258. It also indicates that you probably shouldn't upgrade to 4T9 until you have a full set of the 258 iLevel tier set.
Side Note on the New Eclipse: PTR testers on the EJ forums are reporting that the new Eclipse has been fixed on the PTR and is working as described in the patch notes.