The latest update in my saga, KGHobby responded and asked me to return GT-002, they've reimbursed me for the return shipping cost, and will ship me GT-004 when they receive GT-002 in their warehouse. Apparently, they have a US warehouse in Oregon. So it's not like I'll need to ship it back overseas.
Naturally, I'd prefer not to wait for the replacement, but I understand the arrangement. As they've already sent me the money to ship back GT-002, I see it as a show of good faith. The communication has been prompt and courteous at this stage, so that deserves credit. It at least makes me optimistic that I will receive the proper GT-004 within the next week or so.