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