Socket size not exceeding 1 MB in ios

I'm trying to increase the socket buffer size in iOS but I've only been able to get 1 MB size maximum so far.

I've heard that it can be extended till 4 MB, Can anyone help?

uint8_t buffer[1024*4];
int len;

NSMutableString *output = [[NSMutableString alloc] init];

while ([inputStream hasBytesAvailable])
{
    len = (int)[inputStream read:buffer maxLength:sizeof(buffer)];

    if (len > 0)
    {
        [output appendString: [[NSString alloc] initWithBytes:buffer length:len encoding:NSASCIIStringEncoding]];
    }
}
if (output.length > 0)
{
    [self messageReceived:output];
}