如何强制EditTextPreference只接受四个或更多的字母？

import android.content.Context;
import android.preference.EditTextPreference;
import android.util.AttributeSet;
import android.widget.EditText;
import android.widget.Toast;

public class CustomEditTextPreference extends EditTextPreference {

    public CustomEditTextPreference(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    protected void onBindDialogView(final EditText editText) {
        super.onBindDialogView(editText);

        editText.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {
            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {
                if (s.length() < 4) {
                    getDialog().getButton(Dialog.BUTTON_POSITIVE).setEnabled(false);
                } else {
                    getDialog().getButton(Dialog.BUTTON_POSITIVE).setEnabled(true);
                }
            }

            @Override
            public void afterTextChanged(Editable s) {
            }
        });
    }

    @Override
    protected void onDialogClosed(boolean positiveResult) {
        super.onDialogClosed(positiveResult);

        if (positiveResult) {
            String text = getEditText().getText().toString();
            if (text.length() < 4) {
                Toast.makeText(getContext(), "请输入至少四个字母", Toast.LENGTH_SHORT).show();
                getEditText().setText("");
            }
        }
    }
}

<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">
    <com.example.CustomEditTextPreference
        android:key="example_preference"
        android:title="Example Preference"
        android:summary="Please enter at least four letters" />
</PreferenceScreen>