You can use docker to run multiple Nodejs version simultaneously.
this might be useful for you:
https://nodesource.com/blog/containerizing-node-js-applications-with-docker
https://blog.hasura.io/an-exhaustive-guide-to-writing-dockerfiles-for-node-js-web-apps-bbee6bd2f3c4/
There are other useful resources available on the internet on containerizing Node app on docker.
Docker could be a better choice here but if you don't want to use docker, you can use the nvm run command to target specific versions without switching the node variable:
nvm run 4.8.3 nodeapp1.js
For the other node version :
nvm run 10.15.1 nodeapp2.js
Using forever :
forever start -c /home/ubuntu/.nvm/v10.15.3/bin/node nodeapp1.js
forever start -c /home/ubuntu/.nvm/v4.8.3/bin/node nodeapp2.js