Most tools, weapons and armors have 3 tiers (a couple only have 2 tiers). Knives already have 3 - bone knife, hunting knife, machete. It is very unlikely they will consider adding a fourth tier to knives. If you feel that knives are unbalanced in some way as far as progression, you could explain what is unbalanced and they might be open to adjusting the values if they agree with you.