Choose any 4 options out of 8

Hello, I hope someone can help me here as I've asked this question on other posts and but can't seem to get an answer. 

The problem I'm having is that I have a multiple select quiz slide which has 8 questions. Each has a dropdown option (with two choices - done with layers). The client only needs the learner to answer 4 out of the 8 on the slide. How do I make the slide submittable (and gradable) if the learner clicks submit after choosing 4 options? 

right now, if the learner doesn't answer all 8, the 'you must complete the question' window pops up when submit is clicked. 

Thanks