Based on the provided information and constraints, here's a development outline for the web-based video editor project:

Tech Stack:
- Frontend: HTML, CSS, JavaScript
- Backend: Node.js with Express.js
- Video Processing: FFmpeg (server-side)
- File Upload: Multer middleware
- Video Playback: HTML5 video player

Development Outline:

1. Set up the project structure:
   - Create directories for frontend, backend, and temporary file storage
   - Initialize Node.js project and install necessary dependencies

2. Develop the backend server:
   - Set up Express.js server
   - Configure Multer for file uploads
   - Create routes for file upload and video processing

3. Implement file upload functionality:
   - Create an API endpoint for file upload
   - Handle file size validation (max 1GB)
   - Store uploaded files in a temporary directory

4. Create the frontend interface:
   - Design the layout for video upload, playback, and trimming controls
   - Implement responsive design for desktop use

5. Develop video playback functionality:
   - Implement HTML5 video player
   - Add custom controls for play, pause, and seeking

6. Implement trim selection interface:
   - Create a timeline representation of the video
   - Add draggable handles for selecting start and end points

7. Develop server-side video processing:
   - Set up FFmpeg on the server
   - Create a function to trim videos based on start and end times

8. Implement the trimming request:
   - Create an API endpoint for trim requests
   - Process trim requests using FFmpeg
   - Generate trimmed video files

9. Add download functionality:
   - Create an API endpoint for downloading trimmed videos
   - Implement frontend logic to trigger downloads

10. Optimize video processing:
    - Implement a queue system for handling multiple trim requests
    - Add progress indicators for long-running trim operations

11. Enhance error handling and validation:
    - Implement comprehensive error checking for file uploads and processing
    - Add user feedback for various error scenarios

12. Improve user experience:
    - Add drag-and-drop functionality for file uploads
    - Implement keyboard shortcuts for trimming controls

13. Optimize performance:
    - Implement lazy loading for video playback
    - Add caching mechanisms for processed videos

14. Clean up and file management:
    - Implement automatic deletion of temporary files
    - Set up periodic cleanup of unused files

15. Finalize the application:
    - Refactor and organize code
    - Add final styling touches to the user interface

This plan covers the essential aspects of developing a web-based video editor for trimming MP4 clips, focusing on tasks that can be completed without external web interactions or account creation.