If you refer to your ConfigMap in your Pod this way:

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  containers:
    - name: myfrontend
      image: nginx
      volumeMounts:
      - mountPath: "/var/www/html"
        name: config-volume
  volumes:
    - name: config-volume
      configMap:
        name: some-config

it will be available in your Pod as a file /var/www/html/key4 with the content of value1.

If you rather want it to be available as an environment variable you need to refer to it this way:

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  containers:
    - name: myfrontend
      image: nginx
      envFrom:
      - configMapRef:
          name: some-config

As you can see you don't need for it any volumes and volume mounts.

Once you connect to such Pod by running:

kubectl exec -ti mypod -- /bin/bash

You will see that your environment variable is defined:

root@mypod:/# echo $key4
value1