I made a purchase on a site called
kingkushweed and i sent $3920 bitcoins, now i think i shouldn't have place that much order for a first time, i think i might have been ripped off, Any way to get back my btc sent ?
It's very unfortunate but as the others have said bitcoin transactions are irreversible. So there is no way to get your money back. Best case for you here would be to either contact support at the website, however if you think its fraudulent then it's unlikely that will help.
Apart from that you could file a complaint with local authorities I dont know how effective this would be as it would be dependent on which country you are from as this isn't really effective in a lot of countries with poor ineffective law enforcement.
But might as well make a complaint, even if you can't get your money maybe it will help ensure some one else doesn't make the same mistake