problem in webresponse

[hydroniumf]

I create a downloder program.When I want to download a file , I divide file to 4 parts then program downlads file with 4 threads . ( every thread downloads one part of file) But when 2 threads start ,third and fourth threads dosent continue and they stay in "HttpWebResponse response = (HttpWebResponse) req.GetResponse();" statement until one thread finishes its part . after that third thread continues. I mean only 2 threads run together at ones time. and it happens in response statement. What do you do every 4 threads run together ?

My code:
Uri tval = new Uri(textBox1.Text);
HttpWebRequest req = (HttpWebRequest)WebRequest.Create(tval);
req.AddRange(file_size_ar[i], file_size_ar[i + 1] - 1);
HttpWebResponse response = (HttpWebResponse) req.GetResponse(); ***
Stream str_eam = response.GetResponseStream();


PLease help me