Q. What do I do with my 4 foot high amaryllis plants? How much should I cut them back?
A. That is a large one from your description. The size of this plant may vary with a variety and light exposure. If they are not getting enough light the leaves will be very succulent and long. With adequate light believes should be leathery, stouter and more durable.
You have your amaryllis in good exposure it sounds like with it in the East side with some filtered light during part of the day. Amaryllis does well with half-day sunlight in the mornings. As you've already expressed I'm sure that you amended your soil with compost the time of planting and mulched the bulbs.
Sometimes the flowers need staking because they can get a little top-heavy. They will die back at the first frost. At this time feel free to cut them back to the ground. Cover the bulbs with 4 to 6 inches of wood mulch through the coldest part of the winter.
When all danger of frost has passed go ahead and uncover them and let them warm up. I would fertilize lightly once a month.
On a side note the term Amaryllis in English includes plants that are not truly Amaryllis. The one we see in stores most of the time is Hippeastrum but the trade has called it Amaryllis for so long the name has stuck and it's easier to pronounce. One comes from South America and the other comes from South Africa. One is a little more cold sensitive than the other.Both of them produce gorgeous flowers.