What will be the best cement to cement 4 front emax crowns?

Emax is one of my favorite restorative materials for it offers a superb results in matching the most natural appearance of teeth. However, this material may be fabricated in such way that it will either be more opaque or more translucent. Therefore your dentist will decide which cement will be appropriate to achieve the best outcome while masking darker tooth structures beneath. It is possible to try those cements first to observe the result before the actual bonding. Good luck, Dr. Daniyar

In my opinion, the cement should have nothing to do with changing or opaquing tooth shade that's beneath the crown. That is accomplished with the crown itself. The only consideration with type of cement is whether or not your dentist prefers to bond in the crown or use a traditional cement. 

Hi:
Since the teeth are root canaled, I would suggest an opaque cement to hide the color of the tooth under the crown.
Best,
Dr. Maddahi