Go back and re-read what I wrote. I never once said that the World Bank estimates population, I said it estimates GDP per capita, but it obviously needs sound estimates of national population from the UN to do that.
If the estimate of 3.5 - 4 million for Somaliland is good enough for Senior Economists at the World Bank to conduct their work on calculating GDP estimates, than that is as authoritative source as any.
Remember, this is the same report that found Somaliland has a lower GDP per capita than Puntland. You'll hear no argument from me there. But that is a separate beast than the one we are discussing now.
To be perfectly clear:
Your argument is asinine to put it bluntly. What you are essentially suggesting is that Somaliland simultaneously has both a lower population and lower GDP per capita than Puntland based on the World Bank and UN data shown above. You most definitely failed your introductory Economics and Statistics classes if you truly believe the above statement.