TML is currently undergoing testing for the 1.4+ versions of Terraria, but it is doing so within a public beta that is available to all Steam tML players.

BE WARNED, THE BETA CAN BE VERY UNSTABLE

Simply go to tML within your Steam library, right-click and select "Properties".
Then go to Betas, and click the desired beta from the dropdown.

YOU DO NOT NEED A CODE ANYMORE. JUST LAUNCH AFTER SELECTING.

No! Fear not, 1.4 saves stuff separately, but equally isn't compatible with 1.3 saved characters or worlds. There are ways around this, but we don't recommend them. So please don't ask. To get 1.3 stuff back, simply opt out of the beta the same way you got it initially.

Oh yeah, you can import them and play them in tML with a few simple clicks in-game. You can pick up right where you left off in tML with minimal issues.

Unlike 1.3, tML 1.4 uses the Steam workshop to distribute mods. Simply subscribe to mods like this one, and they should appear on your mods page in-game where you can manage them. This will not work for 1.3, as 1.3 mods are downloaded via the in-game mod-browser, and are saved separately. Removing 1.4 mods is as easy as unsubscribing from them on the workshop.

Please report all issues to the tML Discord server. They can either resolve your issues, or look into patching tML to prevent the issue.

Report it directly to the mod author. Unfortunately that's all you can really do. Disable the mod and report the issue to the maintainer/creator. If you get a response, great! If not, move on. :(