Why did Sky News sit on this for four years and wait until after his death to share with the public?

I don’t think they did. It sounds like they just acquired access to a documentary made four years ago.

Now the question could be asked, why wasn’t it released back then but I suppose a tonne of documentaries get made but never picked up and distributed, it’s just that Navalny’s death made it marketable. I imagine quite a few people are scouring their archives for unused footages - it’s not going to be as marketable in a month.

Of course, the documentary could have been buried by interested parties but a) it wasn’t buried well and b) it doesn’t appear to name names or contain anything explosive so…