What exactly puts Liv on a tier 3 but Nikki on a tier 4?Well, grouping the main roster women in tiers I'd have them like this...
Charlotte Flair, Asuka, Sasha Banks, Becky Lynch, Kairi, Bayley
Ember Moon, Mickie James, Ronda Rousey, Natalya, Ruby Riott
Sarah Logan, Alexa Bliss, Sonya Deville, Naomi, Liv Morgan
Nia Jax, Lacey Evans, Nikki Cross, Mandy Rose, Carmella, Tamina, Dana Brooke, Peyton, Billie
I'd say that the only ones on a similar level as the 4 horsewomen in the ring are Asuka and Kairi. Ember and Mickie are good as is Ruby but they don't quite reach that level. Ronda certainly has the potential to do so but we'll see when(and if) she returns how she continues developing.
Nikki should destroy Liv in a match more times than not by pure aggression difference alone, then take into account that Nikki is a much better wrestler than she gets credit for, but she doesn't always get to display it due to her frantic style.
Liv might be a bit more agile, but that just means she's a glass cannon to me. She normally gets owned against better brawlers and better matworkers alike.
And say what you want about Nia. I do myself. But tier 4 she's definitely not. She's still always dangerous to wrestle for anyone due to sheer size and strength advantage.