Memory foam is a more forgiving material, it will conform to your body shape. However, it's a synthetic material, often made from petroleum. It will offgas toxic fumes during production and usage. These contain VOC aka volatile organic compound, which can be allergens, mutagens, carcinogen, immunotoxic or cause developmental & reproductive toxicity. Due to its density, it traps more heat than most material.
Latex is a more natural material though they are added with a bit of stabiliser. 70% of the latex produced in the world is from South East Asia. That's why Latex should be much cheaper and a better buy here. They are more cooling and lesser toxic than other types of support material.
Spring coils give the resistance to the bed, the layer on top gives the comfort. Some beds are fully latex/foam, some beds use coconut husk as the resistance. If you want something harder, then the spring should be more, the top layer of latex/foam less.
If it's fully latex, you can "sink" in, that's if you like such embracing effect.
You shouldn't go for brand, there isn't alot of ergonomics in bed. Even those zonal support claim don't make a lot of sense, cause who can guarantee which position of the bed are you sleeping and your position. I will suggest that you go to those SEA brands, they give much better value.
Also research shows that people felt the most improvement after changing to a new bed, and a bed can last approximately 10 years. In the research, they gave various bed types. So if the objective is to have a better back care, you will need to plan for regular replacement. Not sure if you want to dump $4k for something that can only last that long. For that price, you can get pure latex. If it's pure latex, most likely it's flipable, meaning you can use both sides and you can use for a longer time. Not saying that it's better though, cause it might be too soft.