The numbers, No 1, No 2, No 4, No 5 etc. are used to identify the different Enfields. The Mk's designate design changes within each No. Such as a Number 1 MkII. The Number 4 would be my pick if I had to call one "best".
Here's a place you learn a great deal about the various models.
http://www.webpages.uidaho.edu/~stratton/en-page.html