The changes between those versions are listed on the History page:
http://www.getrailo.org/index.cfm/download/history/One of those issues highlights that Railo 4.2 switched to log4j which I would guess is at least part of the issue.
It may or not be something you can address yourself (by changing log levels or looking for whether there's a log4j.properties with relevant settings in), but since there are no developers currently working on Railo, this is very unlikely to be fixed at a Railo level. You should look at upgrading to Lucee 4.5 (a fork which the Railo community has moved to). More info at
http://lucee.orgAlso, rather than scanning the log files, you might want to use software like Buglog to index and aggregate all exceptions that occur, so you can get simple summaries, drill down into detail, look at errors chronologically, by most frequent, across multiple servers, etc.
http://bugloghq.com