Since both 7‑Zip (LGPL) and LZ4 (BSD) are open source, modded builds are legal and common. However, they’re unofficial — you won’t find LZ4 in Igor Pavlov’s original branch. Stick to builds from trusted sources to avoid backdoors or broken archives.
LZ4 is optimized for speed (decompression up to several GB/s), not maximum compression ratio.
If you need LZ4 but hate modding: