Using FocusNode in TextFormField Flutter

I believe this is more or less what you are trying to achieve:

void main() {
  runApp(MaterialApp(home: PassCodeExample()));
}

class PassCodeExample extends StatelessWidget {
  FocusNode f1 = FocusNode();
  FocusNode f2 = FocusNode();
  FocusNode f3 = FocusNode();
  FocusNode f4 = FocusNode();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Padding(
          padding: const EdgeInsets.all(8.0),
          child: Row(
              mainAxisAlignment: MainAxisAlignment.center,
              crossAxisAlignment: CrossAxisAlignment.center,
              children: [
                Flexible(
                  child: TextField(
                    focusNode: f1,
                    keyboardType: TextInputType.number,
                    onChanged: (String newVal) {
                      if (newVal.length == 1) {
                        f1.unfocus();
                        FocusScope.of(context).requestFocus(f2);
                      }
                    },
                    decoration: InputDecoration(border: OutlineInputBorder()),
                  ),
                ),
                Flexible(
                  child: TextField(
                    focusNode: f2,
                    keyboardType: TextInputType.number,
                    onChanged: (String newVal) {
                      if (newVal.length == 1) {
                        f2.unfocus();
                        FocusScope.of(context).requestFocus(f3);
                      }
                    },
                    decoration: InputDecoration(border: OutlineInputBorder()),
                  ),
                ),
                Flexible(
                  child: TextField(
                    focusNode: f3,
                    keyboardType: TextInputType.number,
                    onChanged: (String newVal) {
                      if (newVal.length == 1) {
                        f3.unfocus();
                        FocusScope.of(context).requestFocus(f4);
                      }
                    },
                    decoration: InputDecoration(border: OutlineInputBorder()),
                  ),
                ),
                Flexible(
                  child: TextField(
                    focusNode: f4,
                    keyboardType: TextInputType.number,
                    decoration: InputDecoration(border: OutlineInputBorder()),
                  ),
                ),
              ]),
        ),
      ),
    );
  }
}

You can achieve the same by using onSubmitted or even supplying a unique TextEditingController to your TextFields