MC4JLauncher is an indie Minecraft launcher made entirely by me, LG-Smart-TV. Originally, the README, the icon, and parts of the code were made by AI, but I realized that this negatively influenced the app's reputation, so I'm correcting that.
Below, I'll provide an instruction manual for anyone who somehow stumbled upon this project and doesn't know how a Minecraft launcher works, which I find unlikely, but you never know... right?
(It is not included with the launcher.)
Look, I find it unlikely that you'll be able to obtain the source code legally, since the code is being taken down through DMCA claims for being proprietary code. So, I don't care how you obtained it, just don't drag me into trouble if you get yourself into trouble (please). Anyway, I won't provide instructions on how to do it, but obtain it. It can be an already modified version of the code and doesn't necessarily need to be compiled, since MC4JEditor can compile it, but anyway, get it.
Follow all the steps in the MC4JEditor README, create an installation, and save it somewhere for later.
Click on Code and then Download ZIP, or clone it using Git, which isn't really necessary, and place the files created by MC4JEditor inside a subfolder created in the installations folder.
Launch MC4JLauncher.exe, select the installation, and click Play. If you want, right-click the installation, click Extensions, and enable an extension if you added one in the second step.
You can also create multiple installations of different game versions. Enjoy!