I already voted because I feel quite confident with my choices but I guess I'll elaborate even though I'm not going to say anything new.
Scyther isn't played that much because it's not easy to fit on a team (imo), plus it doesn't like the Oma usage rise, but is still very dangerous after a SD. It murders absolutely everything, outspeeds everything, it really only needs a bit of chip and a boost. Nothing likes taking a Slash on a switch. It has that EQ immunity that can always come in handy etc I think it's a strong mon that can happen to have some consistency problems, but it completely makes the 4U cut.
Marco and Lusch covered up why Staryu is 4U-worthy very well. It's an offensive Water option which is better late game than early/mid that you can even pair with your Toise/Oma spot.
Nidoqueen is very strong. I understand why it may not be appealing (twice the same mon etc) but it works out very well in offensive teams. It takes cautious plays, mindgames, and predictions to take care of a Nidoking throughout the game, because everybody plays safely with it. With two nidos you can just blast through the opposite team with one and then apply more pressure with the other one. It's honestly not difficult to play except if your opponent has Blastoise + Staryu because it will always cost a lot of ressources to your opponent, and not that much to you. Also Blastoise, Staryu, Abra, and Moth are what can annoy you, three of those are annoyed by a single Slowpoke that itself can cost ressources. I think the plan with double nido is to paralyse Blastoise/Staryu and then just blast really, which works quite consistently. I have a double nido team that consists of toise lead/king/queen/poke/pory/moth if anyone wants to try it out. It's a strong team. It just takes so much ressources to take down.
As for Fires I already gave my opinion (and you guys covered up very well too) but the stats of Arcanine are significant enough to make a difference ingame, which is enough to make Arcanine 4U worthy (imo). Honestly I think Fire Spin is unreliable, risky, and not a good enough advantage to sacrifice stats for it. Arcanine isn't outclassed, and I honestly prefer it to Dash, but that's personnal. Reading all of you makes me realise I feel that way because I always take sleep/paralysis with my Fire lead, a strategy Arcanine suits better.