Thank you for your email and the photos. From the photos you have provided, I can't see anything visibly that looks like faulty settlement after nearly 4 years of use. Please see the image below of a mattress that does have faulty settlement, please note that this is not one of our mattresses but just for illustrative purposes.
All mattresses settle where you lie the most and this is not a fault but a sign that the upholstery fibres are behaving as they should. When a mattress is made, all the upholstery fibres are completely flat and level with the piping. Adding the piping and the tufts compacts the fibres and creates the domed area. As you sleep and move around on the mattress, the upholstery layers soften and flatten out.
As we are creatures of habit, we tend to sleep in the same position on the mattress, the upholstery layers in this area settle, but leave the surrounding upholstery layers like new. This looks like the mattress has sagged or that there are raised areas on the surface of the mattress. Turning the mattress over on a regular basis and utilising the whole of the sleep surface will help to even out the settlement of the upholstery layers. As a lone sleeper, it is essential to sleep on all areas of the mattress and not just the centre. The upholstery fibres should eventually be flat and level with the piping once again
Please see an article taken from our website.
https://johnryanbydesign.co.uk/understanding-beds/settlement-and-indents-2/
Also below is an article from the National Bed Federation
https://bedadvice.co.uk/mattress-settlement-body-impressions/
Mattresses will always change in comfort over time, and this change in comfort isn't classed as a manufacturing fault. If you feel that the upholstery layers have settled more than the tolerances for your mattress, we can organise to have an independent inspection made of your mattress.
The Company will create a full report, which will indicate if your mattress is just showing signs of natural settlement that fall within industry-allowed tolerances, for the type of mattress, and the length of time it has been used, or if indeed your mattress has settled to a faulty level. If the report deems that your mattress is just showing signs of natural settlement, then there would be nothing further we could do. However, if the report deems that the mattress has failed, then it will be replaced.
Please let me know if you would like me to proceed with organising a mattress inspection.