Seems odd to publish a benchmark against Lucene where your results are "Lucene crashes at 4 concurrent users". Since there are thousands of people using Lucene and products built on Lucene every day, I think the issue may be in your benchmarking code.
There is no secret about the benchmark, its all open source: https://github.com/wolfgarbe/LuceneBench
So, you may be hitting SimpleFSDirectory instead, which does have issues with too many searches.
Could you share the reasons, MMapDirectory did not work for you?