In this case, you can't show the profits as capital gains. You received the coins as salary. So when you sell your coins for fiat, you are required to pay the income tax rather than the capital gains tax. But it is a bit complex. I am not sure whether the tax calculation will be based on the exchange rate at the time of the sale, or the one at the time when you received the coins as salary.
You will be taxed on income tax and on capital gains. Following the above example:
You have chosen the wrong example.
You said that you have sent coins to Mintpal, 4 of them
So , here we have the starting point.
Then you traded on Mintpal and you withdraw 5 BTC.
You would be taxed on 4 bitcoins as income tax (if you earned them on signature campaigns) and on the other 1 as capital gain. At least that's what would happen in my country. It doesn't matter if you traded them or simply the value of bitcoin in terms of fiat grew and when you sold them the value in fiat was higher than when you earned them in signature campaigns.