夫唯弗居，是以不去 What"s the meaning of the first 4 characters?
Nannuo gave a nice explanation. But there are some of the details I don't agree with.

It seems 夫 is 发语词 to me. Normally one doesn't relate it to anything. 

夫唯弗居 => 唯弗居 =>  Only contributing without taking credit.

以 means "because of". 是以 = 以是 = because of it/this/the reason/something.

去 = leave （vanish is also good.)

是以不去 => 以是不去 => Because of the reason above, it (the credit of your contribution) never go away.