This year marks the fourth year of this event being held in Japan.
Japan has hosted this event for four consecutive years.
Hosting this event: means that Japan was providing the event and the entertainment for the rest of the visiting countries as the event is on Japanese land.
Marks the fourth year: means that this event has been going on every year for the past four years in Japan.
consecutive: means each year follows the other without breaks. That for four straight years Japan has held this event.
Hope this helps!
Hosting this event（イベントを開催する）:
日本がその他の参加国を招いて日本でそのイベントを催したことを表します。
Marks the fourth year（4年目になる）:
過去4年間、毎年そのイベントが日本で開催されていたことを表します。
consecutive（連続した）:
「（年が）連続」という意味です。4年連続で日本がそのイベントを開いたことを表します。
This years event is the fourth time Japan has hosted it.
Japan has held this event three time before this years.
The first answer directly tells someone that it is the fourth time Japan is holding this event.
To someone who may not know about the event the second answer explains that Japan has hosted it three times before and this years will obviously be the fourth time.
This will be the fourth time it's he/she's in Japan.
If the event is a concert like a famous musician etc you can just say:
This will be the fourth time he/she has preformed in Japan.
or
He/She has preformed three times in Japan so far! This will be the fourth.
If it is an event like the world cup or the Olympic Games then you can say:
This is Japan's fourth time hosting this event.
or
Japan has held this event three times already.
このイベントが例えば有名なミュージシャンのコンサートなら、こんな風に言えます：
This will be the fourth time he/she has preformed in Japan.
（彼/彼女が日本でコンサートをするのは4回目です）
He/She has preformed three times in Japan so far! This will be the fourth.
（彼/彼女は日本で今までに三回コンサートを開いています。今回が4回目です）
例えばワールドカップとかオリンピックなら：
This is Japan's fourth time hosting this event.
（日本がこのイベントを開催するのは今回で4回目です）
Japan has held this event three times already.
（日本は今までに三回このイベントを開催しています）
If a country holds an event, then it is commonly referred to as 'the host country'. You can apply the verb 'to host' when explaining that any particular event will be sited in a specific country.
"Russia will host the 2018 World Cup."
ある国がイベントを開催する場合、通常「host country（ホスト国、開催国）」と呼ばれます。特定のイベントがある国で開催されることを説明するときは、動詞「host」が使えます。
Russia will host the 2018 World Cup.
（ロシアは2018年ワールドカップのホスト国です。）
This event is celebrating it's fourth year here in Japan.
"This is the fourth annual event in Japan." Annual just means every year or yearly. Here you are saying that for four years you have been having this event in Japan.
"This event is celebrating it's fourth year here in Japan." Typically this means that this is the fourth year in a row that this even is being celebrated in Japan, but it can also mean collectively (over a number of years) that this is the fourth time it has occurred in Japan.
"This is the fourth annual event in Japan."（このイベントは毎年日本で開かれていて今年で4回目です）
"Annual" は「毎年」「年に一度の」という意味です。ここでは、そのイベントが日本で4年間開かれていると伝えています。
"This event is celebrating it's fourth year here in Japan."（このイベントが日本で開かれるのは今年で4度目です）
これは普通は、4年連続でそのイベントが開かれていることを表しますが、何年かの合計で4回目という意味の可能性もあります。
It's the fourth time we have had this event in Japan
This is the fourth time this event is being held in Japan
When you want to explain that a certain event is being held in Japan for the fourth time; then you may explain in the following ways:
- It's the fourth time we have had this event in Japan
-This is the fourth time this event is being held in Japan
開催中のイベントについて「日本で開催されるのは4度目です」と説明したいなら、次のように言えます。
-It's the fourth time we have had this event in Japan（このイベントが日本で開催されるのは4度目です）
-This is the fourth time this event is being held in Japan（このイベントが日本で開催されるのは今回で4度目です）
Japan is hosting this event for the fourth consecutive year.
If you would like to explain that certain event is being held by Japan for the fourth time, you can say:
"Japan is hosting this event for the fourth time."
This sentence explains that it is being held in Japan and that it is the fourth time.
If all four years were consecutive (in a row), you can say:
"Japan is hosting this event for the fourth consecutive year."
あるイベントについて「日本で開かれるのは4回目」だと説明したいなら次のように言えます。
"Japan is hosting this event for the fourth time."（日本でこのイベントが開かれるのは4回目です）
この文では、今そのイベントが日本で行われていて、そしてそれが4回目だと伝えています。
4年連続して日本で開かれているなら次のように言えます。
"Japan is hosting this event for the fourth consecutive year."（日本はこのイベントを4年連続で開催しています）
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