6.9 miles
Assuming that Main Street, Oak Avenue, and Lilac Lane are all straight, you have a right 30/60/90 right triangle. I do object to the phrase "turning slightly right onto Oak" since I wouldn't consider a 60 degree turn to be "slight". But in any case, the short leg of the 30/60/90 triangle is the 4 miles given and the long leg will be sqrt(3) times longer. So
sqrt(3) * 4 miles = 1.732050808 * 4 miles = 6.92820323 miles
So you would need to travel about 6.9 miles on Lilac Lane to reach Main Street.