Hi Caroline,
Welcome to the Mattress Forum! 
[quote]I am in desperate need of a new mattress but kind of frozen in making the purchase…
I bought a Simmons Beautyrest black 4 years ago and although it is not visually apparent (as the warranty requires) the mattress is slumping to the point where it is almost impossible to sleep on and feel rested.
I really don’t want the same to happen with my next mattress. Was it because it has a pillow top? The quality of the foam used? The summer heat?[/quote]
Your second thought was correct. The major manufacturers generally use lower quality materials in their mattress that will tend to soften or break down more quickly than the higher quality and more durable materials that are used by many smaller manufacturers. Therefore, it’s so important to make sure that there are no lower quality materials or weak links in any mattress you are considering. There is more about the quality/durability guidelines that I would suggest in post #4 here.
The first place to start your research is the mattress shopping tutorial here which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly help you know how and why to avoid the worst ones. Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort” and PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists (based on all the parts of your personal value equation that are most important to you).
While I can help with “how” to select a mattress, I can’t choose “what” to buy, as the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort” or PPP (Posture and alignment, Pressure relief, and Personal preferences) or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress) or “theory at a distance” that can possibly be more accurate than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or if you can’t test a mattress in person then your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
While the choice of different types and combinations of materials and components or different types of mattresses (see this article) are more of a preference and a budget choice than a “better/worse” choice (as long as all the materials and components in a mattress are durable enough for your body type), the only way to know for certain whether any specific combination of materials will be suitable for you or which ones you tend to prefer will be based on your own careful testing or your own personal sleeping experience because different people with different body types, sleeping styles, sensitivities, and circumstances can have very different experiences or opinions about whether the same mattress provides “enough” freedom of movement for their own personal preference.
If you provide me with your zip code, I can see if I am aware of any retailers close to our home who offer products using better quality materials for you to try out in person.
Phoenix