ios facebook sdk 4.0 login error code 304

I had a similar problem.

After initialising FBSDKLoginManager I added a line to flush out the data and the (Facebook)Token:

FBSDKLoginManager *loginmanager= [[FBSDKLoginManager alloc]init];    
[loginmanager logOut];

Thus, exactly as the OP asks, “am I missing something”?

Yes, the following standard example code which is seen everywhere, is simply wrong:

-(IBAction)facebookLoginClick:(id)sender
{
FBSDKLoginManager *login = [[FBSDKLoginManager alloc] init];
---- ONE MAGIC LINE OF CODE IS MISSING HERE ----
[login logInWithReadPermissions:@[@"email"]
   handler:^(FBSDKLoginManagerLoginResult *result, NSError *error)
    {
    if (error) {...}
    else if (result.isCancelled) {...}
    else { // (NB for multiple permissions, check every one)
       if ([result.grantedPermissions containsObject:@"email"])
          { NSLog(@"%@",result.token); }
       }
}];
}

you must do this:

-(IBAction)facebookLoginClick:(id)sender
{
FBSDKLoginManager *login = [[FBSDKLoginManager alloc] init];
[login logOut];   //ESSENTIAL LINE OF CODE
[login logInWithReadPermissions:@[@"email"]
   handler:^(FBSDKLoginManagerLoginResult *result, NSError *error)
    {
    if (error) {...}
    else if (result.isCancelled) {...}
    else { // (NB for multiple permissions, check every one)
       if ([result.grantedPermissions containsObject:@"email"])
          { NSLog(@"%@",result.token); }
       }
}];
}

Otherwise, very simply, the app will not work if the user happens to change FB accounts on the device. (Unless they happen to for some reason re-install the app!)

Once again – the popular sample code above simply does not work (the app goes in to an endless loop) if a user happens to change FB accounts. The logOut call must be made.