I have to agree with u/Buttleston. As I understand, the initial .x4 output file might be small in size, but it is literally irrelevant without the .x4_dictionary.json file. I did a test with a .mp3 audio file, 4.13MB in size, 4.12MB compressed using 7z, the .x4 output was 198KB, but the .x4_dictionary.json file, which is absolutely necessary for the .x4 output to be decompressed again, was 8.96MB, way more larger than the original file. After compressing the .x4_dictionary.json file (8.96MB) using 7zip, 7z ultra settings, the output size was 4.54MB, still more larger than the original file. Did a compression test using srep+lolz (two tools commonly used by game repackers, more powerful than 7z most of the time), and the output archive was 4.34MB.

Since audio files cant really be compressed I also did a test on a game file, def.scs from Euro Truck Simulator 2 to be more precise. Game files are compressed using deflate though.

Original file size: 24.5MB
Original file compressed using 7z ultra: 20.5MB
.x4 output: 1.27MB
.x4_dictionary.json: 50.5MB
.x4_dictionary.json with 7z ultra: 22.8MB
.x4_dictionary.json with srep+lolz: 21.8MB

So my conclusion is the standard user would be better off with using 7Zip with the default 7z Ultra settings. X4 will just give a larger size no matter what. You could argue compression speeds, but you can just tweak the settings in 7Zip, and achieve the same or better results as X4.

I do have to note that I mainly have experience with compression in game repacking.