You probably wouldn't say that in English (except, I think, in some regional dialects). You might say "What is your fourth kid called?"
If someone asked me "how do you call your fourth kid?" I'd probably demonstrate the device or method that I used to summon him/her home (if I had four kids, that is).
Is this an English course or a Dutch course? Variations might not be usual and proper uses in English, but the point of the exercise is to show comprehension of Dutch, not mastery of English. My first language is French, all sorts of variations pop up in my brain that fell acceptable to me, and some help my comprehension of Dutch. I don't care that they aren't perfect English translations.
"Which", as an interrogative pronoun, is when asking "what one?" from a definite set. If the set of answers is indefinite, you use "what". Compare:
- "You can choose 10, 22, or 43.677." "Hmm, which should I choose?"
(You could also use "what", but that would mean you could be considering other choices like 33.)
- "You can choose any number you like." "Hmm, what should I choose?"
(You can't use "which".)
Since "hoe heet..." does not imply a choice from a definite set, "what is..." is the proper translation.
You need the possessive form - child's - to indicate that you are asking about the name of the fourth child - "the fourth child's name."
Arguably, "the fourth child name" could work if what you have is a list of names for children - no children, just a list of names, which you refer to as "child names." But even that doesn't work very well.
Please, try to read previous comments.
'How is (somebody) called?' : by which means, with which device do you call/summon this person. (Although to my ears it sounds VERY strange, I would never say it like that, at least not conscioulsy... When I'm tired/distracted I might).
'What is (somebody/something) called?': what is the name of this person/object.
Because that translates to Wie is jouw vierde kind? or something like that (sorry, learner here).
Anyway, they are two different questions.
1) -What's your fourth child's name?
You're asking about the name of the child.
2) -Who is your fourth child?
-The one wearing black trousers.
Here you're asking your interlocutor to point out/identify their fourth child (as if you were looking at pictures or you were at some sort of school gathering, and you want to know out of all of those kids, which one is your interlocutor's fourth child.)
Hope this helps.