在AFNetworking 3或4中，使用GET请求并获得响应对象后，可以通过以下方式获取responseString和responseData。

AFNetworking 3示例代码：

AFHTTPSessionManager *manager = [AFHTTPSessionManager manager];
[manager GET:@"https://example.com/api" parameters:nil progress:nil success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
    // 获取responseString
    NSString *responseString = [[NSString alloc] initWithData:responseObject encoding:NSUTF8StringEncoding];
    NSLog(@"Response String: %@", responseString);
    
    // 获取responseData
    NSData *responseData = responseObject;
    NSLog(@"Response Data: %@", responseData);
} failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
    NSLog(@"Error: %@", error);
}];

AFNetworking 4示例代码：

AFHTTPSessionManager *manager = [AFHTTPSessionManager manager];
[manager GET:@"https://example.com/api" parameters:nil headers:nil progress:nil success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
    // 获取responseString
    NSString *responseString = [[NSString alloc] initWithData:responseObject encoding:NSUTF8StringEncoding];
    NSLog(@"Response String: %@", responseString);
    
    // 获取responseData
    NSData *responseData = responseObject;
    NSLog(@"Response Data: %@", responseData);
} failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
    NSLog(@"Error: %@", error);
}];

在成功的回调中，可以使用responseObject获取到响应的数据，然后根据需要进行转换成字符串或者直接使用原始的数据。