Add 4 as an option of the xs prop in Col

[huyenltnguyen]

Description

In /learn, we have a layout where xs={4} is required:

https://github.com/freeCodeCamp/freeCodeCamp/blob/b234cf80970e2b01a8175e4d9ce3a40ba0c85cbf/client/src/components/settings/certification.tsx#L391

But the Col component doesn't support 4 as an xs value:

ui/src/col/types.ts

Line 4 in 3ee138b

xs?: 6 | 8 | 10 | 12;