My feelings on Black Panther is similar to the Captain Marvel ally: at first glance there is something off about this version of T'Challa. He costs too much (by one), or shouldn't have two consequential damage on both ATK & THW. He should have 3 life and one consequential damage for ATK and THW. He should have only one consequential damage for ATK (or vice-versa).

His 4 hit point could potentially be used to block twice, but that's risky for four cost.

Your most likely play is to tutor your event, ATK or THW once, and chump block (playing your event sometime in that timing), basically what we use 2-cost Allies for. Even then, most 2-cost Allies have some kind of associated ability. That additional 2 cost for a single-card tutor (only from your dsicard pile, not still in your draw deck!) had really better be worth it! The most impactful events are likely also expensive and require a certain amount of setup to be worthwhile, making the timing on the Black Panther ally to be a dicey proposition.

As with Captain Marvel, T'Challa was obviously designed for inclusion with the Sneak Attack Leadership archetype (as he was included with War Machine's deck that just so happens to include the relevant events), He is able to help with Sneak Attack's biggest weakness: getting the right cards at the right time. Whether Sneak Attack itself, or Save the Day/Go Down Swinging, you can help the archetype along by having the right card ready. You can even grab one of those to specifically use with Black Panther himself.

He slightly more playable than Carol, as 4 cost cards are more realistic for inclusion. Still, he must be carefully considered as to what event to attach to him. He will not fit into the average Leadership deck, to say the least.

My rating is a C : potentially effective in niche builds.

Black Panther is an ally that is either too expensive, or suffers too much consequential damage: 4 ER (5 minus 1, the Event you tutor from the discard pile) for either 2 damage and a chump block, or 4 damage, is just a horrible ratio.

The only cost-effective use I can see for this ally is to play him with Sneak Attack, use his ability to tutor either a Go Down Swinging or a Save the Day card from your discard pile, make Black Panther attack or thwart, then use the very same card you've attached to him to discard Black Panther to deal 4 damage (with Go Down Swinging) or remove 4 threat (with Save the Day), since he would've been discarded anyway with Sneak Attack. Damage-wise, such a combo would deal 6 damage (2 for Black Panther's ATK, 4 for Go Down Swinging) for 3 ER (1 ER for himself, 2 ER for Sneak Attack; we do not count Go Down Swinging since it's tutored from your discard pile, as explained above). 2 damage for 1 ER is good, but it's a very clunky combo which requires your deck to be built around it, with lots of cards required just at the right time or in the right place... and you're also wasting a block. Just not worth it.

I simply house-rule him with 1 less consequential damage for both ATK and THW. Otherwise, I don't see any reason to include Black Panther in any of your decks, especially with so many better options out there (especially in Leadership).