Which of these four languages are the most difficult to learn?

Mandarin.

Omg, I can't even stress how freaking hard that language is. There are literally thousands of Chinese characters that one must learn to write and recognize. The pronunciation is hard as well.

I've been taking Mandarin since last semester, and I can only cram so many characters into my head. It's really hard.

Of the three remaining languages (Korean, Japanese, and Vietnamese), I don't know which is the easiest. My little sister took Japanese and she says pronunciation wise, Japanese is easy. She also took Korean and she says Korean is very difficult.

i think mandarin is quite easy to learn.
i've quite a number of malay friends who are able to speak good mandarin.

Hej, those languages you have chosen seem very difficult, to me anyway. I speak Swedish and English too. Those languages you have named do not at all resemble Swedish or English. I don´t want to make you disappointed, a person can really learn a lot if she/he really have the guts to study a foreign language.

I had 2 Japanese friends when I was a child, and surprise, surprise I learned some Japanese, which I of course have forgotten now.

A humans brain is amazing, we don´t even use half of it´s capacity, but when we really want to learn something, I´m sure we can use our brain more efficiently.

I´m sorry I can´t answer your question, it depends on yourself, which of these language you named, you want to learn. Which language do you think, you will have more use of? I don´t know how talented you are in learning languages, but all the languages you have listed sound difficult. I´m sure, though, that you will make the right decision.

Good luck with your choice. You have quite interesting languages to choose from.

Mandarin

I heard that it is easy to learn Mandarin especially speaking it because there is a form of "phonetics" which is called "pinyin".
The writing in China is similar to Japan since the first king of Japan was chinese (influence).
The speaking language bewteen Japan and Korea is similar because Japan used to rule Korea.

Therefore I think it's Vietnamese the mot difficult.

I don't know but the easiest one is esperanto.