In a blog post by Sarina DuPont, Embarcadero has announced the plans for 10.4:

The article includes the plans for the beta, which will be made available to all active Update Subscription customers of Delphi. RAD Studio, and C++Builder. It also highlights the key features we are expecting (no promise, of course) to deliver. Specific to Delphi are:

- Language Server Protocol for Delphi
- Language Enhancements: Managed Records 
- Unified memory management across all platforms 
- VCL High DPI Styles Support 
- VCL Per Control Styling
- New VCL Components
- GetIt Package Manager Enhancements  
- Unified Installer for Online & Offline installations
- Metal 2 GPU driver support (macOS/iOS)
- Apple API Updates
- Windows API Updates 
- LiveBindings Performance Enhancements

There will also be a large number of bugs and issues addressed, along with many smaller by relevant features. The Update Subscription customers beta hasn't started yet, but remember that it will be available only if your update subscription is active.