You would be better off getting the slope using a tapered insulation board. They come in 4x4 pieces and we’ve stacked them as high as 6 inches to get the slope we needed. You can have an1/8, 1/4, or half inch drop over the 4 feet. If you have a more complicated design one of your major suppliers should be able to offer you a design off your measurements to get you to a drain or around a corner. We use beacon sales for this. They also stock standard slopes if you need it without ordering it.
If you stick to the rafters you should be fine toenailing the joists in as long as your above the existing joist. I’d run it by the town inspector to make sure he looks at it first so he doesn’t have you take up the roof to check your framing.