I think it's time for a resole. There does not appear to be anything defective. You have almost worn holes through the middle of sole.
If you want the soles to last longer, add another pair of shoes or two into your current two pair rotation. Otherwise, you'll likely be resoling on an annual basis, or otherwise regular basis. How do the soles of the other shoes look? I'm not sure where your four year figure comes from, but that mark probably will not be reached if you wear the shoes multiple times every week, walking outside and in various weather conditions.
The four year figure I heard about it while researching goodyear welted shoes and I know about the shoe rotation so I never wore them in 2 consecutive days. I used them in the winter (I live in Ireland so winter is just rain) and almost never in summer because they are double-soled. I have similar shoes that look much better than this one and they were worn all-year.
Where can I resole them with Redenbach soles?