Czwarty rozdział jest najciekawszy.

Because czwarty has to agree with rozdział.

Czwarty is an ordinal numeral (like first, second, fourth), but in Polish it behaves very much like an adjective. That means it changes for:

• gender
• number
• case

Here, rozdział is:

• masculine
• singular
• nominative

So the correct form is czwarty.

Compare:

• czwarty rozdział = fourth chapter
• czwarta strona = fourth page
• czwarte pytanie = fourth question

It is an ordinal numeral, but grammatically it acts like an adjective.

That means it agrees with the noun just like an adjective would:

• pierwszy rozdział
• drugi rozdział
• czwarty rozdział

So in practice, a learner can think of it as a number-word that declines like an adjective.

Because rozdział is the subject of the sentence, so it is in the nominative case.

In Czwarty rozdział jest najciekawszy, the structure is:

• Czwarty rozdział = the subject
• jest = is
• najciekawszy = predicate adjective

Since the subject is nominative, the adjective describing it is also nominative:

• czwarty rozdział
• najciekawszy

Both are masculine singular nominative.

In normal standard Polish, jest should be there.

The verb być = to be, and jest is the 3rd person singular present form:

• on jest
• ona jest
• to jest

So:

• Czwarty rozdział jest najciekawszy = standard, complete sentence

You may sometimes see the verb omitted in very informal speech, notes, headlines, or stylized writing, but as a learner you should use jest here.

It is the superlative form of ciekawy.

The pattern is:

• ciekawy = interesting
• ciekawszy = more interesting
• najciekawszy = most interesting

A very common Polish pattern is:

• comparative = adjective + comparative ending
• superlative = naj-
  • comparative

So najciekawszy literally follows the pattern:

• ciekawszy → najciekawszy

Because with ciekawy, the normal Polish form is najciekawszy.

Polish often prefers a single-word comparative/superlative when that form exists:

• ciekawy
• ciekawszy
• najciekawszy

Using najbardziej ciekawy would sound unnatural here.

A good learner rule is:

• if a common comparative/superlative form exists, use it
• for ciekawy, that form is ciekawszy / najciekawszy

Because it also agrees with rozdział.

Just like czwarty, najciekawszy is masculine singular nominative here.

So both words match rozdział:

• czwarty rozdział
• rozdział jest najciekawszy

If the noun changed, the adjective form would change too:

• Ta książka jest najciekawsza = This book is the most interesting
• To opowiadanie jest najciekawsze = This story is the most interesting

Because Polish does not have articles like English a and the.

So najciekawszy can mean:

• most interesting
• the most interesting

The exact meaning depends on context.

In this sentence, English naturally uses the:

• The fourth chapter is the most interesting

But Polish expresses that idea without any article.

Yes. Polish word order is more flexible than English.

The neutral order here is:

• Czwarty rozdział jest najciekawszy.

But you could also say:

• Najciekawszy jest czwarty rozdział.

That version puts more emphasis on najciekawszy or on the identity of the chapter.

Very roughly:

• Czwarty rozdział jest najciekawszy = neutral statement
• Najciekawszy jest czwarty rozdział = the most interesting one is the fourth chapter

Both are correct.

A rough guide:

• czwarty → chvar-ty

  • cz sounds roughly like English ch, but harder
  • w sounds like English v

• rozdział → roughly roz-jaw

  • dział has a soft sound that is hard to match exactly in English

• jest → yest

• najciekawszy → roughly nai-che-KAF-shy

  • stress is on kaw

A useful stress rule: in Polish, stress is usually on the second-to-last syllable.

So the stressed parts are:

• czwarty
• rozdział
• jest
• najciekawszy

Yes, that is a very common clue.

Many Polish masculine nouns end in a consonant, and rozdział is one of them.

Because rozdział is masculine singular, the words describing it must also be masculine singular:

• czwarty
• najciekawszy

This is why the whole sentence has the forms it does.