I doubt, that there will be added with Shadowbringers many more skill tiers, we get to Level 80 with it, we reach soon the traditional max cap of 99 (100) in say around 4+ years eventually or a bit longer until then.
In the end are completely new skills that don't need any number tiers being added anyway for me alot more interesting and there are even many skils, that haven't had any upgrades yet, that I doubt, that those, which are already on tier 4, will receive on the long run much more tiers at all, because so more tiers you add, so more complex become they to implement them into your combat routine of your skill setting, unless you make them always simple minded traited replacements of earlier weaker versions, so direct upgrades.
Its nice, that SE thinks on backing up changes and such stuff always with some kind of lore, but also lore isn't safe from being changed maybe over time, if neccessary, especially when this lore makes only sense in 1 language for the game, when in other localized languages it doesn't work perhaps in this the same way, because there are no numbers everywhere in German, like described before, making the lore from language to language in that case inconsistent.
I think it would be internationally seen better and easier for the game localization, if the naming style of spells would be in all languages the same, using the spelling change-style, that SE kept using from FF8 onwards and try to reduce as much as possible in return the usage of skill tier numbers at all.
They could have done it allso by just namign the skilsl actuall by what they are doing, then we wouldnt need any numbers, nor any spelling changes.
From Fire, Fire2, Fire 3, Fire 4 to actually somethign like traditionally Fire Burst > Fire Pillar > Explosion > Inferno.
By that naming scheme you know by just reading the skill name logically, that a little Fire Burst naturally must be weaker, than an Explosion or a massive Inferno.
It's even not that the combat classes are the worst contenders with that problem I have with this numberic naming scheme - the craftign/collctors in fact are much worser, with their shear endless tiered numbered skills and traits , it just looks so silly ^^ and partwise also confusing and then are some of them role skills, some of them then again not. Definetely something, that needs an overwork in my opinion.