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.