This is a great idea, and we have discussed making the "variables" of the Sitefinity editions more modular to where, for example, you could increase the number of supported email campaign recipients with an add-on vs upgrading to the next licensed edition.
At the current time, the best way to add Forms capability in Sitefinity 4 is via the Standard Edition and higher. This lets you take advantage of the rich, user-friendly layout experience, and it provides a significant value to any client using their site for marketing and lead generation.
the Telerik team
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