‘Inheritance’?

Not quite true. There is the concept of "proper provision" which can apply depending on the circumstances.

But that will be for actual inheritance. Which this isn't.

To repeat what has been pointed out, despite the thread title, this is not a inheritance situation. The OP said the net proceeds of the house sale were divided among the parents and a small amount of money distributed to three of the four children.

Unless I'm completely misreading the original post, the parents are alive and separated. They divided the proceeds between them when the house was sold and the mortgage paid off but gave small sums of money to three of the four children.

If that's the situation and both parents are alive, the fourth child has no right to one cent of that money.

Your post title is "Inheritance", but it doesn't sound like any of the parties involved are deceased. If the house was in your parents' names only, then they are free to distribute any proceeds they make from its sale in any way they see fit, just like with any of their other assets. Generally speaking, they are under no obligation to distribute that money evenly between their children, or to give any particular child any money at all. Inheritance law doesn't apply here if no one has died.

There can of course be any number of unusual edge cases where the fourth child might arguably be owed some money (e.g. because they had some legal interest in the property, such as a lien, or because there was some pre-existing legal agreement between that child and their parent or parents promising them a share of those proceeds, or because their parent owes them a legal debt of some sort, etc.), but absent any of those atypical circumstances, the fourth child has no legal recourse here and wouldn't generally be owed anything.