Hi minah,
Welcome to the Mattress Underground Forum :).
I am sorry you are having difficulty finding the right combination of 1.8lb and 4.0lb memory foam. I assume you are looking for a queen size mattress? These companies are moving to lower density products mainly for cost reasons, they hope to de-spec the mattress and still keep it at a similar price, they also want to not communicate these changes as they know consumers will call them on it in forums like these.
A $900 queen with very good durability is actually a difficult product to find, as normal poly foam costs have risen in the past 3-4 years.
Here is a thread with information about Zotto Sleep. I also think you should look at our trusted members as they have more transparency with these companies.
I know that that Ghostbed, as you can see on the Ghostbed TMU page has 2.0lb foam for the base and they use 4lb memory foam as a comfort layer in two of their three mattresses.
Thank you and let us know if you have more questions.
Sensei