What kind of boglehead is still buying Cash when he is 5m?
Sure, he plays in an advanced position, but if you bought him after GW4, which is when is price really started to go up, you did not get much of a return from him. In fact, only a handful of points more than 4.7 Digne (not that I recommend buying him either, as he's one YC away from an automatic ban).
White, Saliba, Ruben Dias, and almost all starting Newcastle defenders are better than Cash at this price range.
Matty Cash is still a great deal at his current price of £5.0m. Not only is he the only £5.0m DEF in the top five highest points from a defender this season, but he's also a profitable asset. Since I got Matty Cash, I've made £1.0m, and he's expected to increase to £5.1m in a short time. By the way, I do not purchase players based on their team or cost, but rather on the match difficulty level. At the moment he's better than Saliba and White IMO.
The four DEF you mentioned are far more expensive than Matty Cash and their streak of games isn't that good.