O Python possui "itertools", que inclui uma função para executar esse tipo de combinação para você.
combinations('ABCD', 2) returns AB AC AD BC BD CD
para que você possa fazer algo como ...
#!/usr/bin/python3.5
import itertools
output = []
for i in itertools.combinations(['Cr', 'Hf', 'Mo', 'Nb', 'Ta', 'Ti', 'V', 'W', 'Zr'], 4):
output.append("".join(i))
print(sorted(output))
O que produziria todas as 126 combinações e as classificaria para você.
['CrHfMoNb', 'CrHfMoTa', 'CrHfMoTi', 'CrHfMoV', 'CrHfMoW', 'CrHfMoZr', 'CrHfNbTa', 'CrHfNbTi', 'CrHfNbV', 'CrHfNbW', 'CrHfNbZr', 'CrHfTaTi', 'CrHfTaV', 'CrHfTaW', 'CrHfTaZr', 'CrHfTiV', 'CrHfTiW', 'CrHfTiZr', 'CrHfVW', 'CrHfVZr', 'CrHfWZr', 'CrMoNbTa', 'CrMoNbTi', 'CrMoNbV', 'CrMoNbW', 'CrMoNbZr', 'CrMoTaTi', 'CrMoTaV', 'CrMoTaW', 'CrMoTaZr', 'CrMoTiV', 'CrMoTiW', 'CrMoTiZr', 'CrMoVW', 'CrMoVZr', 'CrMoWZr', 'CrNbTaTi', 'CrNbTaV', 'CrNbTaW', 'CrNbTaZr', 'CrNbTiV', 'CrNbTiW', 'CrNbTiZr', 'CrNbVW', 'CrNbVZr', 'CrNbWZr', 'CrTaTiV', 'CrTaTiW', 'CrTaTiZr', 'CrTaVW', 'CrTaVZr', 'CrTaWZr', 'CrTiVW', 'CrTiVZr', 'CrTiWZr', 'CrVWZr', 'HfMoNbTa', 'HfMoNbTi', 'HfMoNbV', 'HfMoNbW', 'HfMoNbZr', 'HfMoTaTi', 'HfMoTaV', 'HfMoTaW', 'HfMoTaZr', 'HfMoTiV', 'HfMoTiW', 'HfMoTiZr', 'HfMoVW', 'HfMoVZr', 'HfMoWZr', 'HfNbTaTi', 'HfNbTaV', 'HfNbTaW', 'HfNbTaZr', 'HfNbTiV', 'HfNbTiW', 'HfNbTiZr', 'HfNbVW', 'HfNbVZr', 'HfNbWZr', 'HfTaTiV', 'HfTaTiW', 'HfTaTiZr', 'HfTaVW', 'HfTaVZr', 'HfTaWZr', 'HfTiVW', 'HfTiVZr', 'HfTiWZr', 'HfVWZr', 'MoNbTaTi', 'MoNbTaV', 'MoNbTaW', 'MoNbTaZr', 'MoNbTiV', 'MoNbTiW', 'MoNbTiZr', 'MoNbVW', 'MoNbVZr', 'MoNbWZr', 'MoTaTiV', 'MoTaTiW', 'MoTaTiZr', 'MoTaVW', 'MoTaVZr', 'MoTaWZr', 'MoTiVW', 'MoTiVZr', 'MoTiWZr', 'MoVWZr', 'NbTaTiV', 'NbTaTiW', 'NbTaTiZr', 'NbTaVW', 'NbTaVZr', 'NbTaWZr', 'NbTiVW', 'NbTiVZr', 'NbTiWZr', 'NbVWZr', 'TaTiVW', 'TaTiVZr', 'TaTiWZr', 'TaVWZr', 'TiVWZr']
Se você quiser "ordenadamente" é só usar ...
#!/usr/bin/python3.5
import itertools
output = []
for i in itertools.combinations(['Cr', 'Hf', 'Mo', 'Nb', 'Ta', 'Ti', 'V', 'W', 'Zr'], 4):
output.append("".join(i))
for i in sorted(output):
print(i)
que dá ...
CrHfMoNb CrHfMoTa CrHfMoTi CrHfMoV CrHfMoW CrHfMoZr CrHfNbTa CrHfNbTi CrHfNbV CrHfNbW CrHfNbZr CrHfTaTi CrHfTaV CrHfTaW CrHfTaZr CrHfTiV CrHfTiW CrHfTiZr CrHfVW CrHfVZr CrHfWZr CrMoNbTa CrMoNbTi CrMoNbV CrMoNbW CrMoNbZr CrMoTaTi CrMoTaV CrMoTaW CrMoTaZr CrMoTiV CrMoTiW CrMoTiZr CrMoVW CrMoVZr CrMoWZr CrNbTaTi CrNbTaV CrNbTaW CrNbTaZr CrNbTiV CrNbTiW CrNbTiZr CrNbVW CrNbVZr CrNbWZr CrTaTiV CrTaTiW CrTaTiZr CrTaVW CrTaVZr CrTaWZr CrTiVW CrTiVZr CrTiWZr CrVWZr HfMoNbTa HfMoNbTi HfMoNbV HfMoNbW HfMoNbZr HfMoTaTi HfMoTaV HfMoTaW HfMoTaZr HfMoTiV HfMoTiW HfMoTiZr HfMoVW HfMoVZr HfMoWZr HfNbTaTi HfNbTaV HfNbTaW HfNbTaZr HfNbTiV HfNbTiW HfNbTiZr HfNbVW HfNbVZr HfNbWZr HfTaTiV HfTaTiW HfTaTiZr HfTaVW HfTaVZr HfTaWZr HfTiVW HfTiVZr HfTiWZr HfVWZr MoNbTaTi MoNbTaV MoNbTaW MoNbTaZr MoNbTiV MoNbTiW MoNbTiZr MoNbVW MoNbVZr MoNbWZr MoTaTiV MoTaTiW MoTaTiZr MoTaVW MoTaVZr MoTaWZr MoTiVW MoTiVZr MoTiWZr MoVWZr NbTaTiV NbTaTiW NbTaTiZr NbTaVW NbTaVZr NbTaWZr NbTiVW NbTiVZr NbTiWZr NbVWZr TaTiVW TaTiVZr TaTiWZr TaVWZr TiVWZr