Depends. Gauthier is better defensively and is probably a better option if you want to use the 4th line to grind down teams. Gauthier and Engvall are 6'5" and well over 200lbs each and can use their bodies to be a real handful for guys in tight. Then of course Clifford as well.
Spezza has the experience, better offensive ability and the right-hand spot to use at center. So if they want that, then he is the choice. However more than likely Robertson is not getting in the lineup. He's 18, with no pro experience, and is very undersized. Unless he really forces Keefe's hand in training camp, he's likely the 10th extra man behind Malgin, Petan, Korshkov, Brooks, Aberg, Marincin, Liljegren, Sandin and Rosen. He just took Agostino and Bracco's spot as an extra, but he's likely nothing more. Both Spezza and Gauthier are likely in the lineup full time.