A couch
I am soft, only if you put something in my specified area for it.
Soft when you fill the cushions with fluff
If you touch me in other areas, I am hard.
the wood beams holding the sofa up are hard
I am warm and large.
very
There are many nouns with me in front of them.
John's couch. living room couch, plastic couch, etc.
When you are on me, you tend to go to imaginary places, only to realize that this is all in your head.
you can fall asleep on one.
I have four feet and a back bone as wide as me, but taller than me.
It has four legs and generally some sort of "spine" down the middle to support it. It will be longer due to couches being longer than height.
You would place a "pain pill" on me.
My parents lose their pills and phones by leaving it on the couch all the time!
The Note:
You can do some dirty things on a couch.