add plugin api version 4 string to Theme model

Signed-off-by: derMart <derMart@users.noreply.github.com>

Author: derMart

lib/Model/Theme.php

@@ -49,6 +49,7 @@ class Theme implements \JsonSerializable
 		Pico::API_VERSION_1,
 		Pico::API_VERSION_2,
 		Pico::API_VERSION_3,
+		Pico::API_VERSION_4,
 	];
 
 	/** @var MiscService */