First, this phrase is constructed with two words:"非常"&"感谢", not four. Second, word's meaning doesn't equal to its elements' meaning adding up, and they can not be partly replaced freely.

非常 very 感謝 Thanks 非常感謝 Thank you very much.

It's really important to know this. Thank you.

In old chinese language, one character is one word. 非 is “not”, 常 is “common”. But during thousand wars between us and northen nomad, we lost two time. One is Yuan Dynasty（Genghis Khan）, the other is Qing Dynasty (Manchu nation). Manchu ran china for hundreds year. They influnce how we speak. Do you ever wonder why chinese language called Mandarin? You know, English language is just call English. France language is called Franch. But Mandarin is nothing looking like China. It's weird, right?That's because the Mandarin is not china traditional language. It's Manchu's version of China language. In China traditional language, one charactor is word, word is one syllable. But in Manchu language, one word is multiple syllable. So when Manchu start to speak Chinese, they use a lot multiple syllable words. It influnced how Beijing people speak. After hundreds years, the Beijing Language (mandarin) was formed. Because people speak different accent. It's hard to communacate. So when the P.R of China was found, government decide to generalize one accent. After ballot, Beijing Language beat Sichuan Language by 1 vote, and become China offical Language. For me, I speak Shandong Language in home, and speak Mandarin is company. So if you understand this history, it may help you understand your question. In China traditional language, 非 means “not”, 常 means "common". But in Mandarin, 非 and 常 won't be used individually, unless you combine them to this "非常“---a two syllable word. Understand each charactor is improtant. For example, internet celebrity is a new word, internet is 网络, celebrity is 红人. So internet celebrity is 网络红人. But as I mationed above, Mandarin perfer two syllable word. 网络红人is four syllable/charactor. So here is the question, how can we short them to two charactor/syllable? 网红，网人，络红，络人？ Eh.. I wrote too many words, there is not enough space to write. If you are intereted, I'll finish it in my article.

if you wan to say 非常感谢，just say "Thank you very much" 非means not , but it have different means when it together with other words, like 非常 ,in there it used as a adverb.