Beginners always get confused because they think about 'magnitude of the number', rather than thinking about 'size of the diaphram opening'.
Since the f/number = FL/Aperture, a 50mm FL with 25mm diaphram opening is f/2 aperture, with 12.5mm opening is f/4 aperture, etc.
What is engraved on the lens is '1/f', which refers to the size of the opening... and
- we see '1/2' (f/2 lens) = FL/2 = 25mm diaphram opening size;
- '1/4' = 12.5mm opening.
Hence the confusion...25mm is bigger than 12.5mm, and 'bigger aperture' makes sense. Yet the number '2' is smaller than the number '4'; so FORGET
OF THE NUMBER!
That the shutter speed 4000 lets in less light than the shutter speed 1000 is not confusing for most folks. The f/number is similar to the shutter speed in the sense that the larger number lets in less light than the smaller number. Both the shutter speed ('1000' or '4000') and the f/number happen to be the denominator in the fraction, so when both numbers get bigger the fraction becomes smaller...bigger number, less light.