Can't grow 2x2 Spruce

Not sure if this is a bug/working as intended/not yet implemented. Please clarify if possible.

While 2x2 spruce trees spawn naturally in Mega Taiga biomes, it doesn't seem possible to grow them either naturally or with bone meal.

I followed the same rules that are used to grow 2x2 jungle trees:
-Four saplings in a square
-No blocks in the 12 blocks around them
-Tried using natural growth (a 1x1 tree spawns)
-Tried using bonemeal (a 1x1 tree spawns)
-Tried growing on both dirt and podzol.