How long to walk on treadmill equal to 4 miles?

It depends what you are aiming for. Do you just want to become physically fitter or do you want to lose weight?

For example, I run for around 8 minutes on 5-5.2,

then I switch to 4 speed 2 incline for 10 minutes,

then I run on 5 speed for another 2-3 minutes,

then I switch to 10 incline 3.5-3.8 speed for 7 minutes,

then back to 4 speed on 4 incline for 5 minutes.

For information about how to lose weight walking on a treadmill, see the page link, further down this page, listed under Related Questions.