There is only one file type that uses the .LZ4 extension:
LZ4 Compressed File
An LZ4 file is a compressed archive created by one of a variety of LZ4 compression utilities, most notably the lz4 command-line utility. It contains one or more files that have been compressed using LZ4 compression. LZ4 files can be decompressed using the lz4 command-line utility and other LZ4 utilities.
In 2011, Yann Collet released the initial version of the LZ4 data compression algorithm. Based on the LZ77 compression algorithm, LZ4 is a notably fast compression system, allowing users to compress and decompress files quickly.
Over time, other contributors have helped refine the LZ4 compression algorithm, and the algorithm's use has increased. For example, Linux systems now natively support LZ4 compression and decompression, allowing users to create and extract files from LZ4 archives using lz4 and other, related command-line commands.
You can extract the files contained within an LZ4 archive using the lz4 command-line utility (cross-platform) or a variety of other LZ4 decompression utilities. All these utilities can be downloaded from LZ4's website (lz4.github.io/lz4).
Updated: July 1, 2020