It's doable. However, Cards and GI are the 2 most competitive fellowships to match into here in the States. A friend of mine did 4 yrs of medicine residency in Canada, applied for cards fellowships in the US, but ended up doing cards fellowship in Canada (same place where he did his residency). Although he was well-published and a smart guy.
Other fellowships with the exception of Cards, GI, and Allergy & immunology, would be less competitive.
