Personally I don't have too much knowledge about the Four-Faced
Buddha, but usually the four-faced buddha is placed under a shrine.
I think maybe people don't feel too good to put the Four-Faced
Buddha at an altar because one or more of the sides will face the
wall. Or you can say in another way, since the Four-Faced Buddha
has 4 faces, must as well put it in an open area so can face people
at all directions. Maybe if you have space and put a shrine in the
middle of your living room it may seem better. (but who has the
space?)
I would also like to hear from others who knows more about this.
This is only my guess.
For me, at my altar is just a statue of Shakyamuni Buddha,
one-sided, the original form which he manifests to preach the
dharma to sentient beings like you and me. It is much more common
to see the common statue of Shakyamuni Buddha situated at a
Buddhist altar inside a monastery.
I don't think they are any rules as to whether the Four-Faced
Buddha should be pay reverence indoor or outdoor. The act of bowing
for Buddhists is an act of reverence. We pay reverence to people
because we respect them, irregardless of where he appears at, not
because of where they appear. But it would be
disrespectful and unappropriate if for example, the buddha statue
is placed in the toilet. Of course we should not do this.
