The main difference between the DiamondClean Smart 9400 and the DiamondClean Smart 9000 lies in the specific functions and accessories they offer. Here are some of the main differences:

- Colours and design: the DiamondClean Smart 9400 is available in black, while the 9000 may have other colour options.
- Accessories and functions: The 9400 might have specific accessories or functions that are not included in the 9000, such as different types of brush heads or a different travel case design.
- Technology and connectivity: Both models utilise advanced technologies, but the 9400 might have updated or advanced functions that are not present in the 9000.

Since I don't have specific information about the DiamondClean Smart 9400, I recommend visiting the official Philips Sonicare website to see detailed comparisons.