インプットでフォーマットつき

mask_text_input_formatter | Flutter Package

Flutter

  var maskTextInputFormatter = MaskTextInputFormatter(
    mask: "#### #### #### ####",
    filter: {"#": RegExp(r'[0-9]')},
  );

を

            child: Padding(
                padding: const EdgeInsets.all(16.0),
                child: TextField(
                    controller: textEditingController,
                    inputFormatters: [maskTextInputFormatter],
                    autocorrect: false,