Shulker's scale attribute cannot be over 3.0.

How to reproduce

1. Summon a Shulker with scale attribute 3.

   /summon minecraft:shulker ~ ~ ~ {Attributes:[{Name:"minecraft:generic.scale",Base:3}]}

2. Try to set it to 4.

   /attribute @e[type=minecraft:shulker,limit=1,sort=nearest] minecraft:generic.scale base set 4

3. The Shulker stays the same size.