I have a follow up question.
Is the 5th knot a chiyuv or is the tallis passul without the 5th knot?
The tzitzis are not technically posul, however a person that does not have the five knots is violating what chazal said to do. See O:CH 11-14 and Mishna Berura 66. Therefore the fifth knot is a chiyuv.