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The Blood Fury Tattoo's Bloodthirsty Strikes feature grants you the below ability:
When you hit a creature with a weapon attack, you can expend a charge to deal an extra 4d6 necrotic damage to the target, and you regain a number of hit points equal to the necrotic damage dealt.
Can you expend more than one charge when you hit with an attack in order to deal 8d6, 12d6, 16d6 damage, etc?
You quoted the relevant rules (emphasis mine):
When you hit a creature with a weapon attack, you can expend a charge to deal an extra 4d6 necrotic damage to the target, and you regain a number of hit points equal to the necrotic damage dealt.
The rule says that you can spend a charge (only one), and does not provide further specifications about spending more charges for increasing the damage.
It is true that if you have the Extra Attack feature, and hence you can attack more than one time, for each attack that hits you can spend a charge for activating the tattoo feature. For example, if you can attack twice and you hit both times, you can deal 4d6 + 4d6 necrotic damage (one for each hit) and you can regain the same amount of HPs.
Some items allows to use more than one charge: the effect of using multiple charges is in the item's description. For example, the Alchemical Compendium reports (emphasis mine):
As an action, you can touch a nonmagical object that isn’t being worn or carried and spend a number of charges to transform the target into another object. For 1 charge, the object can be no larger than 1 foot on a side. You can spend additional charges to increase the maximum dimensions by 2 feet per charge. The new object must have a gold value equal to or less than the original.
Compare the text with magic items that allow you to spend multiple charges. For example the Crook of Rao has a Spells feature that says:
While holding it, you can use an action to expend 1 or more of its charges
For the feature to allow you to spend multiple charges in one use, it would require analogous wording. This pattern of language is very consistent; there are many items in the DMG that use it, like the Staff of Frost, the Wand of Fireballs, and so on.
