It may not seem like it is a huge grade on the treadmill, but a 4% grade is a decent hill outside. I looked at my last few outdoor workouts on my Garmin 305 watch and looked at the grades of hills I was running on, I live in a fairly hilly area so every run seems to have a good number of hills that I can pull this data from.

The grade for running uphill typically varied between 0% and ~7.5%. There were a few hills that peaked at 15%, but these were short and happened very rarely. In general I would say that a 2.5% grade would be a moderately easy hill, a 5.0% grade would be an average hill, and a 7.5% grade would be a fairly steep hill. If you are going to be doing hills that are a half mile long, I would keep it somewhere between 2.5% and 5%, if you are going to do shorter hill repeats, I would bump the incline up to between about 5% and 8%. I hope this helps.

I checked my Garmin as well and for rollers most were at the 6-8% range. (Steepest was also in the 15-16 range)
With the treadmill needing to be at 1% to equate to level ground outdoors, you may want to always bump up the percent on the treadmill by 1 to simulate whatever hill that you want to do your repeats on.