Then, in your logback.xml:

<configuration>

  <property name="LOG_DIR" value="/data/data/com.example/files/logback" /> 

  <appender name="FILE" class="ch.qos.logback.core.rolling.RollingFileAppender">

    <file>${LOG_DIR}/logFile.log</file>

    <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">

      <!-- hourly rollover -->

      <fileNamePattern>${LOG_DIR}/logFile.%d{yyyy-MM-dd_HH}.log</fileNamePattern>

      <!-- make that every 4 hours -->

      <timeBasedFileNamingAndTriggeringPolicy class="com.example.MyTimeBasedTriggeringPolicy">

          <timeMultiple>4</timeMultiple>

      </timeBasedFileNamingAndTriggeringPolicy>

    </rollingPolicy>

    <encoder>

      <pattern>%-4relative [%thread] %-5level %logger{35} - %msg%n</pattern>

    </encoder>

  </appender> 

  <root level="DEBUG">

    <appender-ref ref="FILE" />

  </root>

</configuration>