Most dark/sinister series 4 or 5 episode?

[Rhonda]

Death's Door is recognisable as the sort of nightmare we've probably all had - so I'll go with that. Also The Joker.

[AgentCooper]

It’s funny to me that people often consider “Epic” to be too silly and comedy-forward. I’ve always found it to be incredibly dark and macabre at its core. As Clemens himself has said, it’s basically a snuff film. Poor Mrs. Peel definitely gets an awful lot of episodes (usually written by Clemens) centered around torturing her, I always wondered what that was about!

[Frankymole]

Yes, I didn't find Epic comedic. It has a bleakness to it (though it looks beautiful and colourful). It could have become comic if Peter Wyngarde and Kenneth J Warren had farced it up, but they were experienced and clever enough actors to take it instead into a kind of grim grand guignol, carrying it beyond melodrama into a picture of psychosis that was just believable enough. A rare case of season 5 treading the right line between grotesquery and threat, and as far as you can get from the repetitive "serial killings" episodes that made the season a bit pedestrian.

As to Clemens doing women-torturing episodes, check out his series Thriller for ATV. And the film And Soon The Darkness. It does start to look a bit unhealthy... then again, better these things are explored in fantasy films and TV than in reality.

[QueenOfSin]

There are some that spring to mind, all for different reasons, interestingly enough - I suppose that shows the depth of the writers' ability.
I won't cover those you have both already mentioned, you're descriptions are spot on, but there's these as well:

• Death at Bargain Prices has Kane planning to blow up London with a nuclear bomb, but it's more goofy than dark in the way it plays out.
• The Hour That Never Was has a delightfully spooky ghost town atmosphere as they try to work out what's happened at the air base.
• The House That Jack Built is the classic dark and scary mansion horror film... and draws heavily on Don't Look Behind You.
• Epic has that feeling of impending doom for Emma, but you know she will escape - with lovely recreations of silent film backdrops and sets to add to the atmosphere as well.
• The Joker is the classic dark and scary mansion horror film... and the second version of Don't Look Behind You, sometimes word for word.
• Return of the Cybernauts is more ominous than the original, with Robert Day's direction delivering a note of unease throughout.

The Joker yes definitely, but Epic and Return of the Cybernauts I don't find very creepy or dark. Epic is cartoony - it has the MGM lion roar like the start of a Tom & Jerry episode, a villain with the textbook crazy foreign sounding baddie voice for his evil scheme and you just know Emma will be all's well that ends well at the finish. It's a great episode, macabre and surreal but I wouldn't say it's dark.

Return of the Cybernauts, the man's a really nasty bit of goods without a doubt but he doesn't seem to be menacing or sinister. The Positive Negative guy has a far creepier vibe.