How did the big 4 ASX 200 bank stocks stack up in September?

The S&P/ASX 200 Index (ASX: XJO) slipped 1.4% in September, with a mixed performance from the big four ASX 200 bank stocks.

Commonwealth Bank of Australia (ASX: CBA) was the weakest performer. CBA shares closed out August trading for $170.30 and ended September…