Dit is de vierde vraag.

Dit means this.

In Dit is de vierde vraag, it stands alone as a pronoun: This is the fourth question.

The basic contrast is:

• dit = this
• dat = that

When you put the demonstrative directly before a noun, Dutch often uses:

• deze with de-words
• dit with het-words

For example:

• deze vraag = this question
• dit boek = this book

So:

• Dit is de vierde vraag. = This is the fourth question.
• Deze vraag is moeilijk. = This question is difficult.

In your sentence, dit is not placed before vraag, so dit is the correct form.

Because vier means four, while vierde means fourth.

Dutch, like English, distinguishes between:

• cardinal numbers: one, two, three, four
• ordinal numbers: first, second, third, fourth

So:

• vier vragen = four questions
• de vierde vraag = the fourth question

If you said vier vraag, that would be ungrammatical Dutch.

Because the Dutch ordinal of vier is vierde.

Many Dutch ordinal numbers are formed with -de or -ste, but you have to learn which pattern each number follows. Some common ones are:

• een → eerste = first
• twee → tweede = second
• drie → derde = third
• vier → vierde = fourth
• vijf → vijfde = fifth
• zes → zesde = sixth
• zeven → zevende = seventh
• acht → achtste = eighth

So vierde is simply the correct ordinal form.

Because Dutch usually uses a definite article when you mean a specific noun: the fourth question.

So:

• een vraag = a question
• de vraag = the question
• de vierde vraag = the fourth question

The article de goes with vraag because vraag is a de-word.

Also, with ordinal numbers, Dutch normally uses the definite article when referring to a specific item in a sequence:

• de eerste dag = the first day
• de tweede keer = the second time
• de vierde vraag = the fourth question

Because vraag is a noun with the article de.

In Dutch, every singular noun is either a de-word or a het-word:

• de vraag = the question
• het boek = the book

Unfortunately, this is something you often have to memorize with each noun. There is no single rule that lets you predict it every time.

A useful tip: when learning a Dutch noun, learn it together with its article:

• de vraag
• het antwoord
• de zin

Because it comes before a noun.

In Dutch, adjectives and ordinal numbers usually take an -e ending when they are placed before a noun. So:

• de vierde vraag
• de eerste keer
• een grote tafel

Here, vierde is functioning like an adjective before vraag, so the -e is required.

You would not use bare vierd in normal Dutch.

Yes. It is very normal.

The structure is:

• Dit = subject
• is = verb
• de vierde vraag = complement

So it follows the common Dutch pattern:

subject + verb + rest

Examples:

• Dit is mijn boek.
• Dat is een goed idee.
• Dit is de vierde vraag.

It is very similar to English This is the fourth question.

Yes, in some contexts that is possible.

• de vierde vraag = the fourth question
• vraag vier = question four

The second version is especially common when numbering things, such as in exercises, tests, forms, or presentations.

So both can be natural, but they are used slightly differently:

• Dit is de vierde vraag. = This is the fourth question.
• Dit is vraag vier. = This is question four.

In many classroom or worksheet contexts, vraag vier sounds very natural.

A rough pronunciation guide is:

dit is duh VEER-duh vraakh

A few notes:

• dit sounds a bit like English dit
• is is short, like iss
• de is usually a weak duh
• vierde has vier sounding roughly like veer
• vraag has the Dutch g, which is harsher than English g

The hardest part for many English speakers is vraag, especially the final consonant sound and the Dutch aa vowel.