It's actually good management to keep #4. Cheap ELC, expansion draft exempt and it's a good bet that Tkachuk and McDavid form a lethal combo for the next decade.
The Oilers current core as constructed, just can't win. But because of their ineptitude, they're now in a position to deal some of them (i.e. Yakupov, Eberle and Nugent-Hopkins) to address other needs while still being able to replace their loss with Tkachuk.
Fortunately, neither Eberle nor Yakupov have the value to return a top pairing defenceman, even if packaged together, especially with Chiarelli as GM. While Hopkins has value, moving him would leave the Oilers hilariously inexperienced down the middle, even with a generational talent in McDavid. And the absolute best shot to acquire a quality defenceman is Hall but thankfully they still consider him part of the core and leadership group.