log4j.appender.myAppender=org.apache.log4j.DailyRollingFileAppender
log4j.appender.myAppender.DatePattern='.'yyyy-MM-dd-HH
...

DailyRollingFileAppender appender = new DailyRollingFileAppender();
appender.setDatePattern("'.'yyyy-MM-dd-HH");

Logger root = Logger.getRootLogger();
root.addAppender(appender);