If you now have good air movement through the attic, there's a petty good chance that the frost will sublimate.
Still, what ills the patient? Too much moisture is escaping yoour envelope and getting into your attic.
Attack the source: mainly, proper removal at the source: cooktop, showers, laundry room, etc. Ensure these areas have properly sized and operating vents.
Attack the movement: With only 4" of FG, it's fairly easy. Pull back the FG and seal each and every penetration in your attic floor with canned foam. Electrical holes, plumbing, HVAC chases, even simple gaps in the framing(except those gaps around combustables). How about your attic access? If you have pulldown stairs, you can make a box to cover the trapway out of a couple of sheets of 2" polyisocyanate insulation. Use the same canned foam and duct tape (long live duct tape!) to seal the pieces together to form the box.
In MN, 4" of insulation in the attic may be inadequate. Either after you make your foaming "repairs", or in the Spring, consider blowing more cellulose insulation in your attic.
While loosly blown cells won't stop air movement, they're still better than blown FG at restricting convective air flow, and should you end up with any more water in your attic, cells will also handle it better than FG.