Official comment
Allocation unit size basically tells the drive the smallest block of memory to assign for data being written. If you select 4k blocks and the data is 2k, it will still use 4k. If the data is 6k, it uses two blocks (8k). Larger blocks are beneficial because they can read/write faster than having to address numerous small blocks, but in the case of rekordbox, which does have lots of small files, you may be best using the 4k block size.
That said, I almost always allow the system to use the default when formatting.