Full Idea
Frege says the number four is assigned to the concept 'horse that draws the Kaiser's carriage', but the four horses that drew the carriage did so together, not separately. No horses, not four, fall under the Fregean concept.
Gist of Idea
Each horse doesn't fall under the concept 'horse that draws the carriage', because all four are needed
Source
comment on Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §46) by Oliver,A/Smiley,T - What are Sets and What are they For? Intro
Book Reference
'Metaphysics (Philosophical Perspectives 20)', ed/tr. Hawthorne,John [Blackwell 2006], p.125
A Reaction
They say that Frege stumbles because he is blind to irreducibly plural predicates.