You can theoretically connect the TP-Link Deco E4 to the iPhone hotspot, but there are some limitations and potential issues you should be aware of:

- The Deco E4 can be connected to an Internet source device via Wi-Fi or Ethernet. An iPhone hotspot can serve as a Wi-Fi source, but the configuration and stability of the connection may vary.
- There are reports that the connection between a TP-Link Wi-Fi extender or router and an iPhone hotspot can sometimes be problematic, especially when it comes to creating a server that serves other devices. It may be necessary to adjust specific settings or consult the manufacturer to find the best configuration.
- Make sure that your iPhone and the Deco E4 are configured correctly and that the iPhone's hotspot function is working properly.

Overall, it is technically possible to connect the Deco E4 to an iPhone hotspot, but you should be prepared to resolve any configuration issues.