I was looking at the list of Magic Items and noticed the "Phylactery of Undead Turning". It allows the character to turn undead as if his level was 4 higher than it is. I thought of paladins, which don't get Turn Undead until level 4. If he was wearing this item at say, level 2, would he be able to turn undead like a level 3 Cleric even though he hasn't gotten that class feature yet? And more generally, can this apply to any class feature similar in requirements to this?
By Default, Yes, Bonuses to Level Can Give Access to New Features
If you received a bonus to your effective level, without caveat or restriction, it would allow you to use class features early. To the best of my knowledge, no such straight-up bonus exists, however; they always have some kind of restriction.
Still, if the bonus applies for the purposes of the class feature you want, and the bonus does not require that you already have that bonus, it will work. You will gain the bonus and as a result be able to use the class feature early.
A Simple Example: Initiator Level
The martial adept classes from Tome of Battle use an Initiator Level, similar to Caster Level et al., to determine which level of maneuvers they are allowed to use. Unlike Caster Level et al., however, Initiator Level adds half one’s levels in other classes, and multiclass martial adepts are explicitly allowed to access higher-level maneuvers than they otherwise could at their class level.
Another Example: Ardents
The Ardent class (Complete Psionics) may choose to learn any Psionic Power from its chosen Mantles, so long as it is capable of manifesting that Power. The class does not have a Maximum Power Level as other Psionic classes do, so the only thing preventing them from manifesting higher-level Powers is their Manifester Level. As such, bonuses to their Manifester Level allow them to choose higher-level Powers.
An Extreme Example: Anima Mage
For a really extreme example (and almost certainly an oversight by Wizards of the Coast), the Anima Mage prestige class from Tome of Magic states that it counts as levels in the Binder class for the purposes of the Soul Binding class feature, so that Anima Mage levels stack with Binder levels for using Soul Binding.
The problem? You can enter the class without levels in Binder, and since at no point does Wizards actually reference “existing” levels in Binder (as they do for spellcasting, for example), you end up with a situation where your zero levels in Binder plus your levels in Anima Mage equals your level in Anima Mage.
It’s dumb, almost-definitely a mistake, and should not actually be allowed at the table, but it is a good example of how rules that work the way you want the Phylactery to can be written.
In This Particular Case, and Probably In Most Cases, No
The Phylactery of Turning’s benefit only applies to “a character able to turn undead,” which is not the case if you’re a Paladin below level 4. You therefore don’t get any bonus, and without the bonus you do not have sufficient level to use Turn Undead.
If the item had been worded differently it might have allowed the Paladin to use Turn Undead.
Simply removing the clause about “a character able to turn undead” would do it: something like “you may treat your class level as four higher for the purposes of using Turn Undead.” For a Paladin 2, that would allow him to Turn Undead as a Paladin 6. For, e.g., a Rogue 2, however, it allows him to Turn Undead “as a Rogue 6,” which is to say not at all, since Rogues never get that feature.
Almost Everything Is Worded to Prevent This
As mentioned in the section on the Anima Mage, spellcasting prestige classes always refer to “existing levels” in spellcasting classes. So too do manifesting classes, meldshaping classes, invoking classes, and so on. The Phylactery of Turning is the not the only bonus to Turn Undead that specifies that you must already have Turn Undead. Many explicitly even say “this feature enhances an existing ability to Turn Undead. If you do not already have the ability to Turn Undead, this feature does not grant it.”
So it’s very rare to find a bonus that allows this kind of thing to happen. Even though it is not the default, almost everything explicitly prevents it.
And, Unless It Explicitly Says Otherwise, It Was Probably a Mistake
As also mentioned in the Anima Mage section, because this kind of thing is as rare as it is, most players and DMs assume it is a mistake whenever something does leave it open to behaving this way. Unless the rules explicitly state that the bonus can be used to access new, higher-level class features, you’ll likely have a hard time using them with most DMs. Even the Ardent, which is pretty clear about only requiring the ability to manifest the power and conspicuously lacks a Maximum Power Level column that other psionic classes have, is sometimes debated. I’ve never met anyone who allowed an Anima Mage without at least one level in Binder.
I would say no, simply because the character is not yet able to turn undead.