Blackmetal weapons need tier 7 forge to upgrade to level 4. Armor need level 5 at max. That's it.

An early case example that people frequently experience is with troll hide armour; It can be crafted and upgraded quite a ways once you get deep into the bronze age, however, you cannot get it to to lvl 4 until you get a lvl 5 workbench for the trollhide.

Annoyingly, the last upgrade you need for the workbench by this point requires both Iron AND obsidian, so you're liable to have lvl 4 iron armour before lvl 4 trollhide.

Higher tier items need a higher tier forge.

Crafting tier does not equal item tier. There are items that require a tier 7 forge to upgrade to tier 4.