Yes, to the stairs. We have a step stool that Gracie uses, Boo's too 'manly' to use it, he just jumps.
Neither have every offered to chew our leather furniture...wood is a different story. Since Gracie is a pure blood Beagorkie (a Yorkie, Beaver, Goat), wood is one of her most favoritest things to chew. She'd even pull pieces off of the wood fence in the back yard!
At 4 months of age, I would strongly suggest contain him somewhere when you aren't home and able to watch him. At 7-8 months of age my DH was tired of gating Gracie in the kitchen and left her loose a few times...bad decision. Even though she is through her teething stage, she is still a Beagorkie and still, aaaa cough-cough, loves to chew on things. Her Daddy agains gates her when he leaves.
So getting furniture will probably be ok, you just need to keep a close eye on him because of his age.
1) Gracie the Beagorkie on our leather couch. Look how sweetly she's laying there...must have a full tummy from eating the leg off the end table.