At the point in the game we are currently in, there is no particular reason for Dark Knight to not get Tier 4 elemental magic. Due to a lack of native MAB, Magic burst Bonus(And poor options for gear concerning Magic burst bonus), weapons well suited for nuking, lack of magical accuracy, no relevant buffs to magic damage(Scarlet Delirium is not reliable), low native MP pool(Entropy isn't exactly an ideal WS to be using most of the time) and the fact that every other nuking job has access to T5 nukes...

I feel like if you added T4 nukes to one of Dark Knight's gift categories, it isn't going to be game breaking. The prohibitive recast/cast time on T4 nukes, along with their high mp cost(For Dark Knight) means that it's very unlikely that chaining nukes for TP with Occult Acumen will be viable. For starters, there ain't much Occult Acumen gear out there(I believe Drk can hit 1.85% per MP spent). If you went hardcore on Store TP, you could get 80-90 Store TP in that same occult acumen set, which means Thunder IV would give you right around 650tp. While that does seem high, consider that you have to sacrifice all of your magic damage gear, so the spell will do nearly nothing damage wise. Also consider that with the same gear, a Dark Knight will likely swing their weapon twice and get over 500tp, for no MP cost and quite probably dealing better damage.

For very high level content people would consider this an alternative way of getting TP, but in that instance consider that chances are good that your WS will still miss, so you likely will not be making skillchains this way. Dark Knight has no good Magical WS to fix this issue, nor is using Sneak Attack viable since it is on a one minute timer. Occult Acumen is a neat, niche trick and that's about it.

So why do I even think Dark Knight could use them?

Apex mobs, and that's literally it. Ninja and Corsair are almost certainly the best Apex Monster jobs at the moment, they completely invalidate the need for traditional nuking jobs(Which you don't need anyway, but they can help). Resolution lacks good skillchain properties, and Scythe WS provide utility but are underwhelming on damage. Being able to nuke on a burst in proper MAB gear will provide some decent damage.

All this said, I don't think T4s should come for free. I think they ought to be placed at either the 100jp or 550jp level.