Yes, you can call it a second language.
More generally, ordinal numbers in English can be applied to multiple things in various contexts (mainly metaphorical ones), even though it seems unintuitive.
John Lawler gave the example of "second-class" in a comment. Another example is the word "third-party": it's used to imply an alternative to the first two "parties" (people or organizations), but there may be multiple third parties.
It may make more sense if you think of it, not in terms of ranking all of your languages, but only of ranking the language you're talking about relative to some reference set. For languages, the usual reference set is your native language or first language. The language you're talking about is different from your first language, so it is a "second language." The existence of other languages that you speak besides these can be considered irrelevant. Similarly, when talking about a "third party," you're comparing it to the reference set of the two main parties. The existence of other, non-main parties is not taken to be relevant.
This is just one option. If the context makes the existence of these other languages relevant (for example, if you're talking about possible beneficial effects of learning a second language for learning subsequent languages) you could use the terminology "third language."