I like this idea seeing how it is in OSRS the only issue I can think of would be that in Alora Varrock armour 4 is incredibly easy to obtain where it is much more challenging in runescape. With the same affect as prospectors jacket nobody would spend the time to actually obtain that piece of the gear and deem it completely worthless. Maybe have it so you must buy the Prospectors jacket and use it on the Varrock armour 4 to obtain the boost.
Full support on this one. Even if Varock armour 4 is very easy to get it’s not a reason this shouldn’t be added. This feature exists in OSRS and my suggestion is to make it way harder to obtain.