O trilho circular demora quatro horas.

Because the subject is O trilho circular, which is singular.

• o trilho = the trail
• circular = circular
• together: O trilho circular = the circular trail

The verb demorar agrees with the subject, not with quatro horas.

So:

• O trilho circular demora quatro horas. = The circular trail takes four hours.
• Os trilhos circulares demoram quatro horas. = The circular trails take four hours.

Even though quatro horas is plural, it is just the time expression, not the subject.

Here, demorar means to take (time).

So demora quatro horas means takes four hours.

This verb is very commonly used in Portuguese to talk about duration:

• A viagem demora duas horas. = The journey takes two hours.
• Quanto tempo demora? = How long does it take?
• Demora muito? = Does it take long?

It can also mean to be slow / to take a long time in other contexts, but in this sentence the idea is specifically duration.

Because Portuguese often expresses duration directly after the verb, without a preposition.

So:

• demora quatro horas = takes four hours

Literally, it is closer to takes four hours than to lasts for four hours.

This is normal in Portuguese with time expressions:

• O filme dura duas horas. = The film lasts two hours.
• A caminhada demora três horas. = The walk takes three hours.

English often allows both takes four hours and lasts for four hours, but Portuguese usually just gives the length of time directly.

Because in Portuguese, adjectives often come after the noun.

So:

• trilho circular = circular trail

This is the most natural order here. Portuguese does sometimes place adjectives before the noun, but that often changes the style, emphasis, or even the meaning. In this case, trilho circular is the normal straightforward order.

A few similar examples:

• um percurso longo = a long route
• uma caminhada difícil = a difficult walk
• um caminho estreito = a narrow path

In this context, circular means the trail forms a loop: you normally start and finish in the same place.

So it does not necessarily mean a perfectly round shape. It means circular route / loop trail.

This is a common word in hiking and walking contexts in Portuguese (Portugal):

• percurso circular = circular route / loop route
• trilho circular = circular trail / loop trail

Portuguese uses the definite article o / a / os / as more often than English uses the.

So O trilho circular demora quatro horas sounds natural and complete.

If you removed the article and said Trilho circular demora quatro horas, it would usually sound unnatural as a normal full sentence, unless it were part of a heading, label, or very compressed style.

In Portuguese, articles are very common before nouns:

• O hotel
• A praia
• O percurso

Because the number is quatro, so the noun must be plural:

• uma hora = one hour
• duas horas = two hours
• quatro horas = four hours

This is the normal number agreement pattern in Portuguese.

Compare:

• demora uma hora
• demora quatro horas

In European Portuguese, trilho is the usual word for trail / path, especially in hiking contexts.

For a learner, it is useful to know this difference:

• European Portuguese: trilho
• Brazilian Portuguese: trilha is very common

So in Portugal, O trilho circular sounds perfectly natural.

You may also see related words such as:

• caminho = path / way
• percurso = route
• rota = route

But trilho is a very good, natural hiking word in Portugal.

Yes, levar can sometimes be used to mean to take (time), but demorar is especially natural here.

Compare:

• O trilho circular demora quatro horas.
• O trilho circular leva quatro horas.

The first is very natural and common when talking about duration. The second may also be understood, but demorar is often the safer and more idiomatic choice in this kind of sentence.

You are also very likely to hear levar with a person as the subject:

• Levamos quatro horas a fazer o trilho. = We took four hours to do the trail.

So:

• the trail takes four hours → O trilho demora quatro horas
• we take four hours to do the trail → Levamos quatro horas a fazer o trilho

A simple learner-friendly pronunciation guide would be:

oo TREE-lyoo seer-koo-LAR duh-MOH-ruh KWAH-troo OH-rush

A few useful notes:

• trilho has the lh sound, like the lli in million for many English speakers.
• h in horas is silent.
• o in O is often pronounced like u in connected speech in European Portuguese.
• European Portuguese often reduces unstressed vowels, so the real spoken form may sound more compressed than the spelling suggests.

If you want a more natural European Portuguese rhythm, the sentence may sound something like:

u TREE-lyu circu-LAR dmu-RAH KWA-tru O-rush

The exact sound varies by region and speaking speed, but the biggest challenge for English speakers is usually the reduced vowels.

It can be used with both.

With a thing as subject:

• O trilho demora quatro horas. = The trail takes four hours.

With a person as subject:

• Eu demoro quatro horas. = I take four hours.
• Nós demoramos quatro horas a fazer o trilho. = We take four hours to do the trail.

So demorar is flexible. What changes is the subject:

• the trail takes four hours
• we take four hours to do the trail

Both ideas are natural in Portuguese.