What does raise a red flag for me is what happens after a year. 'Market value', how much is that? It seems they assume that business is going to grow fast, but there's no sure way to tell. Remeber: they're in stealth mode so they probably have no revenue to speak of at this point. If business isn't going well in the future you're likely stuck with the same salary and 4% of nothing, at best.
As for the net salary, you can use www.nettolohn.de (in German). I took the liberty to calculate the net wage from EUR3500,-, it will be somewhere around EUR 2100,- monthly.
Having said that, Berlin is a great city, amazing atmosphere and a nice startup culture. If I had to choose any European city to build a startup from, Berlin would be my first choice. With 3500 euros monthly, you should be able to rent a decent place, buy food, go out and save for the occasional trip to Romania.