Are there any changes that I need to take using the new jars? Do I need to redesign my components?
http://community.jboss.org/wiki/richfaces40componentsdevelopmentkit - yes CDK was redesigned much and will not be backward compatible with 3.3. Now we working at detailed documentation with kickstart samples, but unfortunatelly need some more time to done. will announce as far as some draft available.
I saw that Calendar is not in the release notes - but what about the renderers?
All the components moved according to timeboxed releases plan and moved as finally released. We do not release components code partially.
It's a little a bit desperate to re-design all our components upon a new release (which took us almost a month to develop).
Would it be possible to create it in a way that in richfaces5, custom components of richfaces 4 will have need no changes to work in version 5?
To make development faster / tools friendly and compatible - were one of the main goals of current redesign. we removed all proprietarry stuff like jspx teamplates, config files and so on and deisgned new CDK using standard extension points (annotations, faces-config extensions) and designed new xml templating with public schemas. I'm sure that migration of your current code and moving forward with new components - will much more easier for now.
Our main goal was to simplify community usage - and we have no plans for such complete redesign/compatibility breakages in future for sure.