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Challenge a Jump in Challenging Chain Wild Draw 4 UnoFour's

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Challenge a Jump in Draw 4 Uno

I was playing Uno, and the card was a yellow 4. As I didn't have a yellow nor a 4 in my hand. I played the draw 4 card. Now should I have been challenged, I would have won the challenge. But my friend behind me in the rotation, jumped in with another draw 4, making it a +8. Now that he has played a +4, I am no longer challegeable, but would you be able to challenge him? I don't think you would be able to, since it wasn't his turn so technically he didn't have to play anything, plus it was a jump in. So can you challenge a jump in draw 4 card played?