Differential Pair Reference Plane Coupling
First, 98 ohms is probably close enough to 100 for 9 out of 10 applications. 

Second, I expect most flex vendors to support narrower than 5 mil traces (although I'm not sure how much it impacts reliability). 

Third, Are you sure 4.6 is the right Dk for your flex material? Checking one product, polyimide might be closer to 3.4. 

Fourth, you can increase the spacing between traces to increase Z0.