A 6 belt to 4 belt balanced splitter for Factorio 0.15.x and earlier versions.
This tutorial covers the construction and workings of a balanced 6 belt to 4 belt splitter. While a 6-to-5 or 6-to-6 belt balancer would have a higher throughput, it’s difficult to build such a splitter in a compact space. 6-belt splitters are ideal for trains as each car can only be accessed by 6 inserters per side. This 6 to 4 belt design is ideal because it is only 7 spaces wide at its largest point. Since each train car has 1 space between it and the next, you can copy this design into a blueprint and paste them side-by-side without worry.