I apologize for the formatting issue in my previous response. The fields should not be empty. Here's the corrected YAML file for the Nginx deployment:

```yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: nginx-deployment
  labels:
    app: webserver
spec:
  replicas: 4
  selector:
    matchLabels:
      app: webserver
  template:
    metadata:
      labels:
        app: webserver
    spec:
      containers:
      - name: nginx
        image: nginx:latest
        ports:
        - containerPort: 80
```
This YAML file is a complete and valid Kubernetes deployment configuration. The fields are not empty; they define the necessary metadata, labels, selectors, and container specifications for creating an Nginx deployment with four replicas.
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