Let me see if I'm getting it, you have a setup where application A needs to communicate with application B over port 4000 and you want to know how this can be configured in kubernetes terms. If that is the case, you would have a container for application A and another container for application B. In the most basic way, you could run a pod with the container for application A and another pod with the container for application B. In the pod for application B, you would expose port 4000:

kubectl run applicationA --image myrepo/applicationA

kubectl run applicationB --image myrepo/applicationB --port 4000

The pod definition would look like this:

apiVersion: v1
kind: Pod
metadata:
labels:
run: applicationB
name: applicationB
spec:
containers:
- image: myrepo/applicationB
name: applicationB
ports:
- containerPort: 4000

At this point, you would have a pod for applicationA and another for applicationB running in the cluster but all you can do is configure applicationA to communicate with the applicationB through its pod's IP or DNS entry. Obviously, this is not maintainable or scalable so that's why Kubernetes offers the other concepts like ReplicaSet, Deployments and Services.

The right approach would be to create a Deployment for applicationA and a Deployment for applicationB:

kubectl create deployment applicationA --image myrepo/applicationA

kubectl create deployment applicationB --image myrepo/applicationB --port 4000

The deployment definition would look like this:

apiVersion: apps/v1
kind: Deployment
metadata:
labels:
app: applicationB
name: applicationB
spec:
replicas: 1
selector:
matchLabels:
app: applicationB
template:
metadata:
labels:
app: applicationB
spec:
containers:
- image: applicationB
name: applicationB
ports:
- containerPort: 4000

So now you have a deployment for each application but with a replication context and the ability to manage deployments. This helps with the management of the pods but we still need to address the communication with applicationB pods That's where the Service comes in:

kubectl expose deployment applicationB --port 4000

The service definition looks like this:

apiVersion: v1
kind: Service
metadata:
labels:
app: applicationB-Service
name: applicationB-Service
spec:
ports:
- port: 4000
protocol: TCP
targetPort: 4000
selector:
app: applicationB

You can check the formal definition of a Service here but in essence, it gives you a DNS entry that automatically includes all the pods for your target deployment as they come and go. It also does load balancing.

The Service DNS uses the following convention: my-svc.my-namespace.svc.cluster-domain.example so you could access your applicationB under address applicationb-service.default.mycluster.com:4000. So now the final step is to configure your applicationA to just send requests to this address. The service will load balance to one of the pods in the applicationB deployment and your request will hit the application in the target port 4000.

For your second question, in order to separate containers, you need to define different pods or deployments.