There’s a good writeup for getting started with and using SVN to download the Flex 4 source code, so if you’re a beginner, that’s a great place to start. If you’re a heavy user of SVN, then the below is going to be useless. But if you’re semi-new to SVN and hoping to get started quickly with the Flex 4 source code, I hope this makes it as easy as possible by limiting the number of steps.

Currently I’ve got an SDKs folder on my computer where I store various SDKs for use. To check out the source code, open up the Terminal and use the following steps in whichever directory you want to store your Flex 4 SDK:

mkdir flex_4
cd flex_4

We’ve got the latest Flex 4 SDK inside of the trunk folder of the main Flex 4 open source repository, so that’s all we want to check out. Note the ‘.’ at the end of the command, that’s just going to make sure we put this in the folder we’re currently in:

svn co http://opensource.adobe.com/svn/opensource/flex/sdk .

That may take a little while. Once that’s done you’re ready to roll. Now if you want to keep that build up to date, you just need to go into that flex_4 directory and use one little command:

svn up

That trunk folder in Open Source is constantly being updated with the latest Flex 4 checkins, so it can change often. Once you’ve got your brand new Flex 4 SDK downloaded you can start using it in Flex Builder by adding multiple SDKs using these instructions.