You decide what you think it is first - either a loan or a gift. You then file your tax return based on what you have decided it is.
If the IRS later audits your tax returns, and the tax return and laws match with the facts, then you're fine.
It sounds like you think it is a personal loan. If so, it will not count against the $14K annual gift limit, because it isn't a gift. If you want to make sure of this (perhaps you are worried the person won't pay you back, and thus it will look like a gift), you could certainly write "personal loan" in the memo field of the check and/or send them an email stating that it is a personal loan with no interest.(*)
Another obvious way to show that it is a loan and not a gift is to keep track of the fact that they repaid you the $4K later.
In practice, the IRS audits very few people, and those that they audit at lower income levels I believe they take a look at your tax return and make sure it's correct. They don't do a total colonoscopy level examination of all of your financial accounts and transactions to search for tax evasion. So if you can support all of the numbers that *are* on your tax return, they won't care about the numbers that *are not* on your tax return.
Now if you're money laundering or something, then the FBI or police may get interested, but I don't think the IRS does.
(*) Technically, if you loan someone money and don't charge them interest, then the non-payment of interest from them to you becomes a gift from you to them, but in practice with family loans I don't think anyone cares too much.