Should a 234 tree Nodes link to their parent as well as their children?

When Implementing a 234 tree should the Nodes link to their parent as well as their children or should they link only to their children? I know that how i do it is ultimately up to me. But in practice (in your experience) do they normally link back to their parent?

Any tree which requires traversing upward from a child should reference the parent, otherwise you have no way to traverse upward.

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