Xenogears is technicaly part5...
now i heard the Xenosaga team were planning to make a Xenosaga parts4-6 using a different character arc, since Shion's story ends at part3, depending on the total sales of the first 3parts... sadly i have not seen that come to fruition so i guess the total worldwide sales were not enough to convince them to make the second trillogy, of course due to legal reasons parts4-6 were not gonna be the ID/Fei arc, but were rather gonna focus on a different set of characters in the Xenosaga universe...