While I believe PE on Air NZ is good (and have heard good reviews, but not flown it), is it really worth the $4000 to you?
$4000 will provide upgraded meals during your entire time in the US, or upgraded accommodation for most/all nights.
You should get a couple of hours to stretch your legs in Auckland before heading on the next leg to the US (I believe that leg is about 12 hours?). This isn't as bad as a direct Sydney to San Francisco flight which is more like 16.
Of course the decision comes back to you, and yep the trans-pac flights are a bit of a PITA when it comes to sitting in Economy. But that's only ~16 hours of your entire journey.
There are additional Frequent Flyer benefits for flying PE, but of course they need to be factored into it if you are particularly keen. And you will most likely be more refreshed when arriving in SFO over having been in Economy only (and possibly out through immigration/customs quicker etc)
Anyways, just my 2c. Others may have different comments (and for me, if it was 1 pax or 2 pax, then probably my opinion may have been different).