Hi abhilash,
couple of suggestions to you..
one, if you would have been in either hyderabad and chennai you could have attended the Adobe developer summit which had introductory to detailed sessions on Flex 4.
if you have missed out on the sessions you can have a look at the presentations that speakers would have uploaded by now post these developer summits, go visit this site to get more information
http://endtoend.in/apps/forms/adobe/DevSummitNovDec09/home.html
you can also check the presentation that mrinal gave during the summit on flex 4 components that will give out some idea to you about new things in flex 4
http://weblog.mrinalwadhwa.com/2009/12/01/custom-components-in-flex-4/
i have given a session on skinning in flex 4, you might want to check out that presentation as well
http://saurabhnarula.com/blog/presentations-on-skinning-in-flex-4-flex-debugging-tips-and-tricks
second, flex 4 as a platform has evolved, its difficult to write a all the changes that have happened, to start of with you can check out the spec sheets at this url
http://opensource.adobe.com/wiki/display/flexsdk/Gumbo
to answer your questions
what is main differences to flex builder 3.0 and flash builder 4?
if you are asking about the IDE, then the apart from the name change ( flash builder) there are several enhancements in code hinting, break points, debugging, profiling features, network monitor, data model drives development etc etc.
about the SDK changes, well there are several and you need to go n read them at the adobe open source url that i mentioned above, for one the architecture has evolved and changed and now is more robust and developer and designer friendly.
Do they want to anyway merge time line based editor flash and flex!!
in future?
no this has not been the intention as far as i know, the editors are still different, coz the underlying technology is different, timeline based editors stay with flash suits and code based editors stay with flex.
also i saw flash catalyst, to develop RIA's, is this flash catalyst
has something to do with flex related application?
Well in the entire Adobe Flash platform stack, yes it has to do a lot and its has great deal of importance, wilh flex yes, it will be significant to use catalyst with flex, the intention behind this is the developer - designer work flow, with catalyst you would have more control over this workflow and you can streamline the process and your applications and you both become mature and productive :)
post flex 4, the catalyst would be placed as a tool between the designer and developer, where in a designer can work on the designs separately and the developer can make use of it, the new language FXG facilitates this :), go ahead and read more about this.
adobe mentions future portfolios, buisness and internet websites will
be more RIA and all mostly belong to concept of flash catalyst?
post flex 4, yes to a certain extent, because it would easy to conceptualize your applications in catalyst, also transformation of your static designs from an image( built using photoshop etc) to an application would be easy.
I hope i was able to give you a sneak peak into flex 4, and i hope my responses were able to answer your questions.
Thanks
Saurabh Narula
http://saurabhnarula.com