Is it possible to extend a +4 colour choice card?
The short answer is no.
If the previous player places a +4 colour choice card or draw four colour choice card, the player in turn must immediately take 4 cards into his hand. If the previous player has a +2 card or a +4 colour card in his hand, he is not allowed to lay it.
Deviating rules must be established as a house rule in advance of the game.