It doesn't seem to matter what I give setLayers, the resulting Snow will always be 1 level high

block = chunk.getBlock(x, y + 1, z);
block.setType(Material.Snow);
BlockData data = block.getBlockData();
if (data instanceof Snow) {
Snow snow = (Snow)data;
snow.setLayers(4);
}

(note: thanks again for all the work on 1.13. I am slowly porting my plugins to support it. With that in mind, it is entirely possible that I am doing something wrong. I looked around for an update function but couldn't find it – BlockState had one for example)