What mix 1:2:4 means, when you see such specification it simply means, ratio of cement to sand and to granite, that is, 1 pan of cement mixed with 2 pan of sand mixed with 4 pan of granite.

Note: 1 bag of cement contains 2 head pan, so if you are to mix 1 bag of cement at once, it will be 1 bag of cement with 4 headpans of sand mix with 8 head pans of granite.

Another example is mix 1:3:6, which means 1 pan of cement mixed with 3 pan of sand mixed with 6 pan of granite.

Note: 1 bag of cement contains 2 head pan, so if you are to mix 1 bag of cement at once, it will be 1 bag of cement with 6 headpans of sand mix with 12 head pans of granite.