I havent seen any new T4 Soul yet and by what I expierienced so far, the pages drop with a hilariously sad chance.
Still unsure if you need all 5 steps to unlock the T4 Souls, but I guess this will be the case, which means you need a total of 5 books which translates into 500 pages.

The shop offers 3 batches of pages.

25 for 100 Raid Medals each
25 for 150 Raid Medals each
1000 for 200 Raid Medals each.

Going by this, unless you are one hell of a lucker and hoster/MVP-Master, this is gonna take some time until you got those collected.