I've had almost the exact same experience here. No real oddities occurred in-game besides the game locking for another player while in multiplayer.
The world worked perfectly fine and saved as normal, but when I tried to play singleplayer, it simply gave the "Unable to load world", nor could it find a backup. In the worlds folder (in the documents directory), the main .wld folder seemed to be nonexistent, though a backup did exist, somewhat luckily. The backup loaded by removing .bak from it, however, it's... odd. The backup folder seems to be at least a week old, as it was last modified on May 18th (The world was created on the 16th). My character data is a-okay, as well as the stuff in the piggy bank (which I assume is associated with player data? So no matter).
The backup world is linked below, though I doubt it'll be much useful. The original world file seems to be gone with the wind, at least locally, though it was on Steam cloud, so maybe there's a way to get it back? I've no idea. Regardless, I
would love to get the world back, so I do hope it's possible, otherwise, a friend and I are forced to get everything we lost back and such.
It does seem that this is a major issue of 1.4.0.4 based on reports though, which isn't good. If any other info is needed or what else was done in the world that may give help, let me know.
EDIT: I forgot to mention that going from singleplayer to multiplayer worked fine (if I'm remembering it updating before playing multiplayer later on).
Backup world file
