"I do not know the fourth one."
Translation:Toho čtvrtého neznám.
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I went with Toho čtvrté neznám, thinking since I didn't know what one was, it should be neuter. My answer was wrong.
Now, that I read here the implied object is člověka, requiring the masculine. But, could the implied object not also be dítě requiring čtvrté, or holky requiring čtvrtou?
Or, is there some principle of Czech I'm missing?
Edited a few minutes later: Because of the toho!!! That's the principle I'm missing! Yikes!
To čtvrté neznám. Tu čtvrtou neznám.
Or, Ten čtvrtý neznám to complete the set.
It is understood without translating the "one". From Czech point of view, it's strange why English actually needs it there (probably because of missing declensions - Czech declensions make it often more clear what exactly I mean).
You can say "Toho čtvrtého muže/člověka neznám", but that has better English translation "I don't know the fourth man/person."
PS: You're mixing masculine and feminine numerals here. "Čtvrtého" is masculine while "jednu" is feminine ending.