FR4 is a printed circuit board layout and design tool. The goal is to do for PCB design what OpenSCAD did for STLs. Boards are described using a programming language which generates a model of the design. This model can then be rendered for display, serialized to a Gerber file set for manufacture, or simulated for design verification.
All you should need to build FR4 is an up-to-date Rust installation.
cargo build --releaseFR4 is licensed under the GNU General Public License Version 3. Note that this license does not preclude using FR4 for proprietary projects. However, it does require that any redistribution of FR4, in original or modified form, be provided to users with the corresponding source code and that derivative works retain the same license.