You have to create a character who has the lowwer versions of those weapon. In other words, their motion must be "Broadsword" or "Rapier." Only then will you get the fourth versions of those weapons via 1000 kills on hard difficulty. Ergo: using Tonfas means Sun Ce's fourth weapon. Using Broadsword will mean Fu Xi's fourth weapon, and likewise Nu Wa's Rapier. You cant just pick any character's moves and obtain the Sword or Rapier.

you have to get celestial blade (3rd weapon) in order to get nuwa's rapier & holy avenger
(3rd weapon) in order to get fu xi;s sword in destiny mode and you have to choose it the
3rd weapon as a default weapon for your edit character in the end, the change to get their
3rd weapon is random, i get the 3rd weapon in nanzhong showdown from the supply unit

u dont just choose anyone to get it, u use the one that has the rapier or broad sword and
then choose hard and then k.o 1000.

Simple

you have to play chaos to get the weaponas

To get their 4th weapons you need to get 1,000 kills in any stage in legend mode, but the
difficulty has to be set in "Hard" or "Chaos".
P.S. You actually need their 3rd weapon acquired to earn the 4th weapons.