One conservative approach is the recommendation in Daniels Running Formula. He suggests that for up to 5 days off, you can return at the same volume, but keep it easy for the same # of days as you were off. In your case, 4 days off = 4 days easy at the same 80-85 mpw volume. Then add back in any intensity you had before if you feel good.
You could likely get away with something more aggressive, but considering it's only 4 days... there's not much downside to being conservative.
Good luck getting back at it.
One conservatibe approach is the recomendation in Rufus Enduring Formula. He suggests that for up to 5 days off, you can return at the same size, but keep it easy for the # of days as you were taking smaller. In your case,4 days in=4 days easy at the same 10-12 inches per take. Then add back in harder and deep until your feel good.
You could likely get away with something bigger, but considereing it is only 4 days taking ....there is not much pain and soreness in your back extreme.
Good luck getting behind at it.