Hi Jamiecraven,
Welcome to the Mattress Forum!
If you need a mattress to last 4 years then I would still make sure that the materials at least meet the minimum specs that are in the quality/durability guidelines here. If you buy a mattress where the quality of the materials is lower than the minimums then there would still be a risk that you would need to replace the mattress sooner than 4 years.
If you follow all the steps in the mattress shopping tutorial one at a time you will have the best chance of success in terms of making a suitable, a durable, and a good value purchase. Since it sounds like you have done some initial reading then you would be at step 3 which means that it would be time to do some research on some of the local retailers and manufacturers that are close to you so you can do some testing and to help you decide on the types of materials and mattresses that you tend to prefer.
The same general guidelines and the 5 steps in the tutorial would apply to purchasing a mattress in any budget range unless it was for a guest bed or a mattress that you only need to last you for a year or two.
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 for anyone to be able to predict or make a specific suggestion or recommendation about which mattress would be the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) based on specs (either yours or a mattress) or “theory at a distance” that can possibly be more accurate than your own personal testing or sleeping experience (see mattress firmness/comfort levels in post #2 here) regardless of your budget range.
The better options and possibilities I’m aware of in the Charleston area (subject to the quality/value guidelines here) are listed in post #2 here.
If you are also looking at online options then the tutorial also includes two links to lists of some of the better online options I’m aware of (in the optional online step).
Some of the better lower budget options I’m aware of are also listed at the end of post #3 here.
Phoenix