Hi wk5h, and welcome to The Mattress Underground.
Glad to have you 
here.
Sorry to hear you are having trouble finding a replacement for your old mattress. It can be quite a challenge to replicate the feel of your 12-year-old mattress when it was in its’ prime. It may be useful to include some of your stats (height, weight, and sleep position(s) for you and your wife) to be able to better guide you. but generally, a split king configuration is a good way to go for sleeping partners with different comfort needs and preferences.
Unfortunately, one issue with the major mattress brands like TempurPedic is their lack of transparency in disclosing mattress specifications you need to know – construction, materials used, and the quality firmness of individual layers – which makes it difficult for consumers to make an informed decision.
[quote] About a month into it, my back was letting me know the 4 was too soft, and I was able to exchange it for a 3.
About a month after that, my wife and I are both now waking up to lower back pain. Nothing debilitating - just what she describes as “tender” in the lower back, and it hangs with us both in the morning for a couple of hours. [/quote]
Purple is using a layer of buckling column gel while it is a quality and durable material and some people may do well with … instead of getting firmer when you compress it more deeply like all other foam materials … buckling columns start off firmer when it’s not compressed and the gets softer when the columns buckle under pressure.
If you are interested to see why you may have gotten off the track with selecting this mattress you can see is more info from Phoenix articles about buckling column gel in this article and in post #2 here and a forum search on " buckling column gel " will bring up more comments and feedback about it.
In your search for a new mattress it would greatly benefit you to start off with a few basics and take a look at the mattress specifications you need to know to compare the quality of materials and components to the mattress durability guidelines here. Also, we have a Mattress Shopping Tutorial you can find information to help in your approach to finding the best mattress for both you and your wife. You definitely have the right idea, as the most successful way to find the right mattress is to do some in-person testing locally.
I took a look at the Saatva Loom and Leaf; as, they are not transparent in the materials used in the layers and they have over 4" of memory foam, While I can’t speak to whether any of the mattresses you selected would be a good “match” for you in terms of comfort, firmness, and PPP … I would be cautious about Saatva’s 12" Loom & Leaf as they tend to use some lower-quality materials than I would normally suggest in the durability guidelines. Saatva does not disclose to me the density of the bottom two layers (see [url=https://forum.mattressunderground.com/t/loom-leaf-memory-foam-mattress-by-saatva]post #2 here[url]). so I would treat them both as 1.5 lb density.
I can’t speak of how well, any of the GhostBed mattresses will suit you or your wife because every person is different and has their own set of needs and preferences, butGhostbedare one of theTrusted members of the site and they have a few well-constructed hybrid mattresses; the Ghostbed 3D Matrix does have dual comfort by use of sensors and a smartphone app which may help if you and your wife have very different preferences, which resembles the comfort control you had with the Sleep Number bed. If you are considering this bed I’d make sure that you are taking into account that there is a gap between the two separate units.
Hope this helps,
Basilio