The original Mystery flag
And now a twisted version...
Which country does this flag belong to?
There is a clue in the question
These given four flags are of the countries Vietnam, Turkey, Yemen, Morocco respectively.
Now the capitals of these countries are Hanoi, Ankara, Sana, Rabat.
By the same logic as the linked question we can say
The capitals names are split in two parts: the first part having the same number of letters as colors in the flag.
A - Red
H - Yellow
R - Green
N - White
S - Black.
The flag shown is Red, Black, Yellow. So ASH. Only capital that starts with those letters is Ashgabat, which is the capital of Turkmenistan
Or maybe it's just a flag of Belgium with colors twisted. Who knows? ;-)