Need to support 15th of ABS pipe
I'm not a plumber, but Is the pipe running against a wall? If so, I would imagine you could just use plastic pipe strapping or plastic clips attached to the wall to support it. As far as how often to apply support, I really don't know. Don't you also need to pitch the pipe 1/4" / foot towards the main stack? If so, then the end with your elbow should be at least 4 foot 3.75 inches from the floor. I'd also imagine you would need a p-trap instead of that elbow. Also, being that far away, won't you need separate venting for that p-trap?
Again, I'm not a plumber, so all this info may be wrong, but this info is what I've gathered from reading various plumbing books and how-tos.