let

x = the time Julius needs to paint the house alone

y = the time Ruben needs to paint the house alone

together they finish the house in 4 days

Julius can paint a house three times faster than Ruben

y = 3x

in 1 day Julius will finish 1/x house, x>0

in 1 day Ruben will finish 1/y house, y>0

in 1 day Julius and Ruben finish 1/4 of the house

1/x + 1/y = 1/4

by solving the system of equations

y = 3x

1/x + 1/y = 1/4

we find

x = 16/3 days = 5 days 8 hours

y = 16 days

the faster painter will finish the job in 5 days and 8 hours.