Kubernetes is an open source project (or even a framework), while OpenShift is a product that comes in many variants. There’s an open source version of OpenShift which is called OKD.
Does OKD-4 provide full access to users to Operators for installing databases like mongoDB, HDFS, elasitcSearch, janusgraph . Or these operators are supported out of the box only in licensed Openshift Container platform and not in open source OKD?
It depends, every operator which is opensource can be used in OKD
But in OCP there are also some operators which coming with a redhat openshift subscription, they are not opensourced, but mostly you will find an upstream operator project on github
example: OpenShift Virtualization Operator 2.6.5 provided by Red Hat
you can look for the upstream opensource project and use this on OKD, because this is redhats DNA: Kubevirt Operator https://github.com/kubevirt/kubevirt/releases/download/v0.43.0/kubevirt-operator.yaml
Basically, if you find an operator on github for the type of application you want to deploy, you can do this on OKD.