You can control the location of log files in Tomcat using the ${catalina.home} environment variable.
Example:
log4j.rootLogger=DEBUG, file
log4j.appender.file=org.apache.log4j.RollingFileAppender
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=%d{dd-MMM HH:mm:ss.SSS} %p - %C.%M(%L) | %m%n
log4j.appender.file.File=${catalina.home}/logs/applicationname.log