I am a little confused on the "limited period offer"-3BTC on first ICO day and 4BTC on the 2nd to 7th day of the ICO.
Is the 3BTC and 4BTC are the prices of 1 PBT?(early bird bonous?)
Correct.
3 BTC is the price of 1 PBT at the first day of ICO and so on

(Yes, this is early bird bonus. Because normal price of 1 PBT is 5 BTC)
Did i understood right, that need 5000btc to start the project? no more or less
Not, excatly. They need between 3000-5000 BTC. I asked them a question about this. There is huge difference(2000 BTC), how much actually they need? As far as I follow, I couldn't get an answer.
But the token sale could be sold out at the first day of ICO. So I assume that they need 3000 BTC at least, if they collect more that will help the company for being more flexible.