Were those four puppies the entire litter?
Black is expected when breeding chocolate to properly pigmented yellow (i.e. yellow with a black nose).
To get chocolate, both parents have to be chocolate factored. In this case, your yellow would have to carry the "b" allele, and you would get half black, half chocolate on average.
To get yellow, both parents have to be yellow factored. In this case, your chocolate would have to carry the "e" allele, and you would get half black, half yellow, on average.
If you bred a yellow that carried chocolate to a chocolate that carried yellow, you would expect 1/4 dudleys (yellow with brown nose), 1/4 standard yellow, 1/4 chocolate, and 1/4 black, on average.
If you bred a chocolate that didn't carry yellow to a yellow that didn't carry chocolate, all puppies would be black. In any breeding, if one parent doesn't carry the recessive allele, the recessive trait is not expected to be expressed in the offspring. That goes for EIC, CNM, PRA, and color.
Amy Dahl