I've got a question. In some reason, I need my friend to keep my $4000 for few months and I was about to wire the money to a friend's account. But suddenly, it came up in my mind about tax issue. He'll give it back in cash.
In next tax report period, is there anything we need to do or we have to pay for tax?
Or I don't need to care about this at all?

What country?
Banks in the US don't report transfers under 10,000.
If you are trying to hide from state or federal benefits, however, it won't work. They look at previous months too for this reason. Same with divorce lawyers and nursing homes, and I can't think of what else at the moment.
(Never mind that fact that your 'friend' could stop being your friend, could decide he needs a loan that he then can't pay back, or he dies... )

I don't get it exactly. What does that mean 'They look at previous months too for this reason.'
The situation is, the money is in the bank in the USA and I'm currently in my home country with the money left in the USA and I asked my friend who is in USA to keep it and give it back to me when he is back to our country.