The 4.5 update should be going live somewhere between this Monday and 8/29.
The most important thing about this release is simply to ensure that all of your clients (Simple Control) and Hub if applicable are updated at the same time. The new version does not talk to the old version and vice versa.
While we do include this warning text in pretty much every release, this one draws a hard line so make that once any part of your Simple Control infrastructure updates, the rest must as well.
In the case of the Simple Hub appliance, you may force the update by unplugging it and plugging it back in. The update will go live relatively simultaneously for everything so once you see it from the App Store, it will be available for macOS and the appliance.
The changes in this release are mostly under the hood improvements for compatibility with the new Simple Control Home product.