You can divide the total hours (120) by the daily hours (6) to get the total days
120/6 = 20 days
Then you would divide the total days (20) by the number of people (4) to get the total amount of people to work 6 hours per day to complete the job
20/4 = 5 days
So, it will take 4 people 5 days to work a total of 120 hours to paint the house.
To check this, you would calculate:
4 people x 6 hours per day x 5 days = 120 total hours