I installed latest sideview utils app in my spunk 4.2.2 installation and ran straight away into a problem. Evertime I try to launch sideview editor it throws 404 errors. 
Any pointers would help
This just means that you have not restarted Splunk after installing the app.
Splunk should give you a notification after installing the app through the Manager section, that a restart is required. I think that notification system in Splunk may not be working on all Splunk's supported platforms, because we seem to be getting this problem reported with some regularity, where something or other doesn't work properly and ends up being just because the server hasn't been restarted.
By the way, you'll find a ton of questions and answers about this app over on answers.splunk.com rather than here on stackoverflow.