I will be going Hong Kong for 4 days vacation

There is no free shuttle bus from Hong Kong International Airport to Kowloon Metro Station, you can take the Airport Express to Kowloon Metro Station directly, and it will spend around 20 minutes on the way. The ticket fare for single way is HKD 105 per person.

Besides, you can also take Bus A20 or A21 to the stop which is near Kowloon Metro Station, and then you can walk to the station. The ticket fare is around HKD 33 per person, but it will take much more time on the way by bus than by Airport Express.