Four Thousand
numeral
You have four and you have thousand. Multiply them together and you have four thousand.
よんせん
This is a jukugo word, which usually means on'yomi readings from the kanji. That's true for the 千 part, but the 四 reading is kun'yomi. You should know how to read this word as long as you've been studying your kanji!
フォロワーが四千人になりました。
I hit four thousand followers.
この山は、およそ四千メートルです。
This mountain is about 4000 meters tall.
四千円で他人の心は、かえないよ。
You can't buy someone's heart for 4,000 yen, you know.