Make one reference and load the other automatically (Entity Reference)
Broadly if your node reference is required your user must to fill it on node creation. So you could try to use hook_node_presave or in the form layer you could have your submit handler to deal with that. Once you get the reference chosen by the user you would need to retrieve the other 4 related references that will be also added as siblings in the multi valued node reference field.
By another hand if you want to make all of this visible in the form before it gets submitted you could use hook_form_alter (in case you don't have your own hook_form) to add a behavior to the node reference field form element and if it has just a checkbox widget you could simply automatically check the other nodes to be referenced using the behavior to react the check box that has been checked.