http://docs.openhab.org/administration/logging.html#logging-into-separate-file
So for zwave it would look like
# Logger - zwave.log
log4j.logger.org.openhab.binding.zwave = INFO, zwave
# File appender - zwave.log
log4j.appender.zwave=org.apache.log4j.RollingFileAppender
log4j.appender.zwave.layout=org.apache.log4j.PatternLayout
log4j.appender.zwave.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss.SSS} [%-5.5p] [%-36.36c] - %m%n
log4j.appender.zwave.file=${openhab.logdir}/zwave.log
log4j.appender.zwave.append=true
log4j.appender.zwave.maxFileSize=10MB
log4j.appender.zwave.maxBackupIndex=10
And add
log4j.additivity.org.openhab.binding.zwave = false
to the # File appender - openhab.log section, otherwise you will have the entries in both log files.