I would almost never upgrade a 4* item to level 16. You'd really have to justify the specific use case.
Generally, I wouldn't even upgrade 5* to level 16, although from your current progression point it may be selectively necessary. Just make sure you choose the proper gear to upgrade - for example, you should pretty much never upgrade a weapon to level 16 on a DEF-based champ, and similarly, you should rarely upgrade a helm/shield to level 16 on an ATK-based champ, simply because they don't really benefit that much from the primary stat increase, and 5* items are ultimately replaceable.
Good candidates for upgrading 5* gear to level 16 are:
Speed boots
ATK/DEF/HP % chest
Crit % / Crit damage gloves
Also, to a limited extent, ATK/DEF/HP % boots and gloves can be useful, although you need to be careful about those, since you're setting your champs up to require specific teams in order to have those work properly.
Gear you should almost always instantly sell are:
Flat ATK/DEF/HP gloves/chest/boots
Also, for your current progression point, you should probably avoid ACC/Resist chest - they're selectively useful, but you shouldn't need them at the moment.