feat(uart): adds Serial4 as a keyword - used by ESP32-P4

[SuGlider]

Description of Change

ESP32-P4 has 5 HardwareSerial Objects.
Arduino IDE highlights Serial to Serial3, but not Serial4.
This PR adds Serial4 as a keyword.

Tests scenarios

Usinf Arduino ide 1.8.9

Related links

None

[github-actions]

	Warnings
⚠️	The **target branch** for this Pull Request **must be the default branch** of the project (`master`). If you would like to add this feature to a different branch, please state this in the PR description and we will consider it.

👋 Hello SuGlider, we appreciate your contribution to this project!

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