When you say stuff like that Kojiro, it makes you seem like you dont understand simple economics.
It wasn't just the quest changed that increased the amount of t4 materia, its the other 3 changes they made
1) all t3 items can now make t4 as well.
2) sb rate on i 51+ items was increased
3) increased the rate of getting t4
Ergo the supply went up 10 fold or more, and is now considerably easier to get, and t4 materia is only good for crafter AF gear, and i70 crafted DL alternative gear (jewlery being the best option). So now there is way more t4 materia then will ever be needed, the demand is way down.
Good luck buying all those materia and then realizing later the supply wont dry up and your stuck holding worthless merchandise that has price droped even further.