Bull's party lost 2.5 players in the first IP. There's a reason they tuned him down a tiny bit for Romero&Juliette, with the notes of "only 4 Edge, and never overcasts".
Season 4 has a handful of dangerous missions, all in the artifacts storyline. The Ork Underground storyline has only 1 tough run in my opinion, namely Silver Platter. The rest are mostly non-combat smartness. Artifacts has Brothers United, Romero&Juliette, and its first Mission is also rather dangerous, what with 2 highly skilled assassins that start talking then try to kill you. I think that without a sniper covering the meet, that one can easily kill the party. The bugs in Brothers United are tough and players had better use flashbangs, smoke grenades and Superior Cover from the pillars in the final fight. I'm not sure how tough the final Mission of Artifacts Season 4 is.
Sprawl Wilds only has Humanitarian Aid as the really tough one, Carbon Copy may also be tough, if you apply the nerfing from Romero&Juliette to Humanitarian Aid and make sure the players play it smart, and use John to balance things, it should be fine. See the topic in the Missions forum about it for more tips.
So tough ones are HA, half the Artifacts storyline and Silver Platter afaik.