There's nothing advanced in designing in more phases.
You just, instead of a 8V, 100A converter, design a 8V, 25A converter and copy-paste it 4 times.
The only thing to add is to make them run in sync with phase shifts, for example 180 deg for 2 phases, 120 deg for 3 phases, 90 deg for 4 phases, 72 deg for 5 phases... you get the point. For this, you need to create the sync signal for the ICs with phase shifts, if you can't find a fully integrated controller IC that does 4 phases, or a 2-phase control IC with 90-deg phase shifted SYNC OUT, which would allow a simple single wire from SYNC OUT to another chip's SYNC IN to be used for 4 phases with two chips and no extra clock generators.