We got our rattie at 4 months too! She was a pet store pup (and spur of the moment! Hits mom). Anyways, she was CRAZY and had zero manners. Jumped, nipped, barked, peed inside, agressive towards dogs, and knew zero commands. We've worked on it and she's gotten 10 times better. I gotta say though, rat terriers are interesting breeds. They really need jobs. Your pup looks a lot like my Cassie. Heres a pic just cause they're so cute:) She's big for a rat too.
image