Ionliosite said:
The difference is that Melmetal is a legendary Pokémon, Pikachu and Incineroar aren't. Also, if Naganadel isn't 4-B then that should be corrected, as all Ultra Beasts should scale to each other.
Nonsense.
1) There are many members of legendary/mythical species depicted, even Mewtwo and the Creation Trio, depicted within Pokemon media. Such as the HGSS event creating another member of the Creation Trio or there being two Mewtwos in the anime.
2) Ordinary pokemon like Grovyle and Lucario have been scaled to legendaries for fighting them (and they got scaled to multiversal for it, not like Island-level or something)
3) Kukui's Incineroar was undefeated, can tank multiple powerful attacks by multiple pokemon at one time and come out unscathed, is acknowledged as the strongest and ace of the strongest trainer in Alola, Kukui, stomped Ash's Melmetal and took Lycanroc, Melmetal and Torracat to defeat. It's FAR from an ordinary Incineroar.
4) Pikachu is constantly considered an outlier of its species, having defeated Regice and Tapu Koko, matching Latios and Zeraora and contributing at least 50% of the power to overpower a Guzzlord. It also overpowered and defeated Guardian of Alola with 10,000,000 Volt Thunderbolt. At the start of PM, it was the only pokemon to stop and even slightly hurt a Lugia, with that Lugia choosing to flee after breaking out of the shock.
5) Pikachu underwent an entire training arc from the start of SM to the end, with its power and speed steadily progressing in each battle against Tapu Koko. It explicitly grew much stronger over the course of SM.
Seeing as ordinary pokemon can scale to legendaries, legendaries are all treated as equal to any member of their species (across even different universes, mediums and writers) and we have several cases of SM Pikachu scaling to 4-B Legendaries and Kukui's Incineroar and Lucario scale to Guzzlord, Naganadel, Melmetal and, by extension, Tapu Koko (due to Lucario handling Naganadel, who could fight Tapu Koko toe-to-toe), there is no reason or rationale for NOT scaling them to 4-B.
Maybe if Ash's Pikachu has some terrible outlier against Scorbunny in the upcoming episode, you could argue it doesn't deserve 4-B. But I really don't see the argument against the rest of Ash's SM Team or Anime Kukui.