In the US, it's going to come down to the number of credits you've acquired, not necessarily the number of semesters you've studied. However, if you had COMPLETED full four semesters and were "on track," I'd call you a junior. If you were still IN your fourth semester, I'd call you a sophomore.
Again, it does depend on the situation, though. A friend of mine, for example, was in her eighth semester and was technically a senior. However, she called herself a junior because she changed majors late in the game and needed to be in school an extra year.
ETA: Cross-posted! I'd check "junior," unless they have some kind of credit-conversion chart available.