WiredTiger added LZ4 support more than two years ago, but MongoDB does not enable it. Why not? LZ4 has better compression ratio and speed than snappy.
I confirmed my experimental patch for lz4 works on YCSB benchmarks as well as other compression codecs. In the experiments, the compression ratio of storage size is reduced from 28% to 15% if I change journal and block compressors in WiredTiger from snappy to lz4. I used a YCSB in https://github.com/mongodb-labs/YCSB with compressibility=10.