Hi Adam, hope you're having a great day. First I should say that I don't know about the Pashto language very well but for three other languages, I can tell you I had this chance to talk with people from Afghanistan and Tajikistan who talk in Dari and Tajik language and we could understand each other without any problem. The main point is that in Iran the colloquial form is different from the written form and in Dari and Tajik language they use mostly the old version of the words in Persian which is close to our written form. So the answer is yes they are mutual languages and three accents of the same language. If there are any other questions I would be happy to help.

No it's different, I am from Afghanistan and my native language is Pashto and official language is Dari, this four language that you mintion have a big difference between them if you want to know this language I can help you.

As a speaker of all of them I can tell you. Farsi, Dari and Tajik are all Persian and mutually intelligeable. Pastho is from a totally different language family, but shares many vocab words with Persian and Arabic. Also Pastho was not a formal language for most of its existence and as such, much of its grammar and spelling is not set in stone.

No that’s not true. There are mutual words though.