This seems like an interesting item. A few changes are necessary.
You write: "they can instead choose to expend a charge from the necklace", but the wearer will generally be unconscious and making death saves when they die, and someone who is unconscious cannot make choices about magic items. As written, the item can only be used if the wearer is killed while conscious (for example by power word: kill). You need to make the item activate automatically, or to clarify that this works even if the user is unconscious.
This item does not specify how long it takes to cast the spell. The casting time needs to be the full hour. If the item can cast resurrection instantaneously, restoring someone's hit points from zero to full, that's too good. (Instant resurrection might not be too good for a martial character, since the -4 penalty to attack rolls would be a problem for them. But it's definitely too good for a spellcaster who can just use spells that don't require attack rolls. Instant true resurrection is definitely too good, no matter who is wearing it.)
This item probably should not be able to hold revivify, because that's a very different spell than the other four, and it creates some very strange incentives. Someone wearing a revivify necklace suddenly feels sad about "merely" going unconscious: if they go unconscious and make their death saves it takes them out of the action for the rest of the combat, but if they fail their death saves the revivify goes off and they're back in the combat with one hit point.
(If you wanted an item that could get someone back into a combat, that should probably be a different item, perhaps one that would cast an emergency healing spell if someone goes unconscious.)
Consider adding some discussion of how easy it is to notice when one's defeated enemy is wearing this necklace and it is casting a raise dead spell on them. Is it completely unnoticeable until the final stage when they stand up? Mostly unnoticeable unless you examine carefully and notice their wounds are healing? Or does the necklace glow and start chanting in Celestial? This question is very likely to come up in practice, so it's better to answer it rather than make the DM issue a ruling.
With these changes, the item becomes one that will bring you back to life an hour after you died. Basically it's very similar to a scroll of raise dead, except that you don't need the cleric to be standing there casting it. This is not unbalanced.
The effect of this necklace is actually not that great. If you fall off a tower and take lethal damage, sure, the necklace will bring you back. But if you fall into a pool of acid, or you get devoured by a hungry monster, or you get killed by brigands and then they loot your body and take your stuff, the necklace won't save you from any of that. But it's still a nice bonus for the occasional time it saves your life.
High-level groups might want to get one of these necklaces to give to their cleric. If the whole group is wiped out, the necklace will bring the cleric back and the cleric can raise the rest of them. Other characters would probably be happy to have one, too.
The rules for magic item rarity are not super consistent. I think it's correct to call this a "legendary" item simply because it's capable of using a ninth-level spell effect (true resurrection). But it would be reasonable to imagine a variant of the item that only casts raise dead; I think it would be fair to describe this item as "rare" since it only casts a fifth-level spell effect and it doesn't even do that very often.
