Semplice 3 and Semplice 4 are two different products, and are not compatible with each other.
If you decide to purchase Semplice 4, you will need to make a new purchase (it does not work like an upgrade) and start fresh on your site . That means you can continue using your old Semplice license separately, if you wish.
We've collected more information about Semplice 4 compatibility right here.
And please make sure to check our pricing model to understand what you can expect from us in the future.
Thank you for supporting Semplice and helping us make it better everyday. We can't wait for you to try out Semplice 4!