We will announce more information about different Sitefinity 4.0 editions with the release of Sitefinity 4.0 RC. A feature list of the community edition should also be announced for at that time. The CE is expected to arrive shortly after the official release of Sitefinity.
The SDK is a free product which will help you develop modules and other plugins for Sitefinity. It will contain mainly project samples, guides and project templates. This means that you will be able to use it to develop plugins for any edition of Sitefinity 4.0
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