First question - Is it possible for a human to conceptualize a new color they've never seen before, just by thinking about it? I've heard a few people claim they could, but it was "wasn't easy".
Second question - If a 4th primary color could be seen (or imagined), would that increase the number of possible colors 100-fold, to about 100 million? We can see about 1 million colors now. Would a 5th increase it to ~10 billion? If so, could the brain even handle this much color variety?
By the way, some people may already see 4 primary colors (thus 100 million colors), due to an rare mix of genes. Also, certain animals see 4 primary colors normally.
Personally I think that would be the most amazing thing, to see colors never imagined. It would be like waking up from a black and white world.