you got acceptable life out that chain at 4000 miles.
Do you monitor chain wear with the Shimano guage ? a worn chain will wear out the cassette
ONCE in my cycling career I had a chain do this. Still got me home with careful riding.
Stuff does break. I replace chains when they start to show wear. The Shimano chains are exceptionally good.
2500 miles is an acceptable life, at least to me
/markp
