But you do not realize FY4 will not be enough also to stop such attacks. The front armor's main section has around 30cm of different ceramics and compositive materials to give over half meter of steel equivalent. Also the spacing between make it look like 40cm or so and filler ingredients in between. The purpose is to ask penetrator rod to puncture through newer layers everytime. With just steel, it actually can cut through like butter and rip ahead of rod. Nowadays some Korean and German L55 new penetrators can rip through and melt its way if double warhead stream. From above no tank can survive attack. Putting FY4 will be no different just heavier against Spike missile or Javelin. But FY4 compared to FY1 may be able to withstand some old RPG attack or penetrating rifles that's all.
I think whoever bought this VT-4 decided there is no difference for purposes of this tank to put heavier and more expensive FY4 on the roof. They expect purposeful designed top attack missiles will win and only chance to defeat is using GL5. For rifles or machine gun and RPG then FY1 is enough.
Anyway Russia, China, South Korea, and Israel all think defence system is way to defeat missile. Penetrator rod is best defeat using ERA according to Russian and Chinese decision because we prefer lighter tanks which can travel faster with longer range.