Continental 4Seasons vs 4000S
Tyre management is tricky. I also notice the gashes and cuts in the tyre. Also if you take your tyre off and look really closely (while bending the rubber back and forth) you'll find heaps of glass and metal embedded in the tyre. If you were really keen to extend the life of your tyres you can go through and pick out all the junk you can find.

I personally replace my tyres when the frequency of punctures gets too high. Over time it happens more regularly until I get sick of it and buy new ones.

If you want a longer lasting tyre, I've had good experience with Continental Gatorskins. I generally run 4000S's on the front for the amazing grip, and Gatorskins on the back for longer life.