Update readme changelog

Author: mikejolley

readme.txt

@@ -85,6 +85,25 @@ Release and roadmap notes available on the [WooCommerce Developers Blog](https:/
 
 == Changelog ==
 
+= 4.0.0 - 2020-12-07 =
+
+#### Enhancements
+
+- Dev: Change register_endpoint_data to use an array of params instead of individual params. ([3478](https://github.com/woocommerce/woocommerce-gutenberg-products-block/pull/3478))
+- Dev: Expose store/cart via ExtendRestApi to extensions. ([3445](https://github.com/woocommerce/woocommerce-gutenberg-products-block/pull/3445))
+- Dev: Added formatting classes to the Store API for extensions to consume.
+
+#### Bug Fixes
+
+- Checkout block: Prevent `Create an account` from creating up a user account if the order fails coupon validation. ([3423](https://github.com/woocommerce/woocommerce-gutenberg-products-block/pull/3423))
+- Make sure cart is initialized before the CartItems route is used in the Store API. ([3488](https://github.com/woocommerce/woocommerce-gutenberg-products-block/pull/3488))
+- Fix notice close button color in Twenty Twenty One dark mode. ([3472](https://github.com/woocommerce/woocommerce-gutenberg-products-block/pull/3472))
+- Remove held stock for a draft order if an item is removed from the cart. ([3468](https://github.com/woocommerce/woocommerce-gutenberg-products-block/pull/3468))
+- Ensure correct alignment of checkout notice's dismiss button. ([3455](https://github.com/woocommerce/woocommerce-gutenberg-products-block/pull/3455))
+- Fixed a bug in Checkout block (Store API) causing checkout to fail when using an invalid coupon and creating an account.
+- Checkout block: Correctly handle cases where the order fails with an error (e.g. invalid coupon) and a new user account is created. ([3429](https://github.com/woocommerce/woocommerce-gutenberg-products-block/pull/3429))
+- Dev: Refactored and reordered Store API checkout processing to handle various edge cases and better support future extensibility. ([3454](https://github.com/woocommerce/woocommerce-gutenberg-products-block/pull/3454))
+
 = 3.9.0 - 2020-11-25 =
 
 See release post [here](https://developer.woocommerce.com/?p=8234)