There's the adventure packets for Beyond the Wall and Other Adventure, each one is a short adventure and needs no prep because the motivations of the various NPC and the resolutions of the adventure are rolled when the PC create their characters with their playbook.
"Duergar raiders have attacked a secret gnome outpost. During a desperate rescue mission, adventurers encounter giant snails, a net full of gnomes, an octopus, a sealed scriptorium, death, a ruined birthday party, a berserk construct, a haunted letter, and much more."
The Twilight Game in Galder's Gazetteer is a great 5th level one-shot. Haven't run it yet, but been doing more one-shots recently, and I'm active on their Discord, and between my read and what I'm hearing, it runs great in about four hours.
My go-to one-shot used to be my conversion of an old DUNGEON adventure, which I reworked/renamed Hunt for the Archdruid as a 3rd level one-shot. I've run it twice (well two and a half times), and as long as you keep the pacing snappy it fits within 4 hours just fine.
I recently ran my conversion of an old Chris Perkins' adventure, "A Wizard's Fate" (DUNGEON 37), and we ran out our 3-hour clock in the dungeon, but my guesstimate was that if we had another hour, and I'd exercised a bit of a stronger hand with framing the scene, that it would have fit in 4 hours just fine. Also, I ran it online, and games tend to be a bit slower when played online, at least IME.
I've also been wanting to try out Cat and Mouse from Kobold Press. It is a 1st level adventure with a nice little story with a funnel structure – narrow at the beginning, but opening up to more choice towards the end – and there are three parts (which always translates well to running a one-shot. However, as much as I like it, I think it would take some work to streamline it for a 4 hour one-shot session.
Planning one running another old Chris Perkins adventure, Redcap's Rampage, online as a one-shot in September. Like most of the DUNGEON adventures, it's requiring a pretty strong editing hand to conform to a one-shot format (and I run just 3 hour games when running online). Generally, I'm noticing that dungeons as "room-by-room exploration" elements will run out the clock like nobody's business, so I'm mostly reinterpreting the ruins at the end as simply the terrain of a final fight (if it comes to that).
I strongly recommend making a Five Room Dungeon (as distinct from a mere five room dungeon). I've found that model typically gives a two to three hour play session, and given that convention games often get interrupted for various reasons, this should be about right.
OMG this is a gold mine! I gave away all my Dungeon magazines (and Dragon) since I needed space and had the Dragon CD rom, but they didn't have a CD version of Dungeon. So this is like an early Christmas present lol