How tall should my tomato plants be now?

It depends on what kind of soil is in your yard. If you had them in potting soil, that's made to encourage plants to grow. Sometimes the dirt in the yard doesn't have the nutrients potting soil has. If they're only 4 inches tall, they probably don't have a large root system yet. You might dig down about a foot all around them and transplant them again. I put my tomatoes into Home Depot buckets (two buckets stacked together). I drilled holes in the bottom of the top bucket and drilled a 3-inch diameter hole in the middle of its bottom. I also drilled some holes into the sides of the top bucket about four inches from the bottom. I set the top bucket into the bottom bucket, put a plastic cup into the bottom of the top bucket, and filled it with a mixture of potting soil and fertilizer into the top bucket. I then put the tomato plants into the dirt mixture. The roots will grow down into the soil. I water about every other day, using a "mist" setting. Don't just dump water into the bucket. When you see water coming out of the holes in the sides of the bucket, you'll know you've watered enough. I got tomato plants when they were about two inches tall in little peat moss pots. They're now almost three feet tall, and full of blooms. I also spray the leaves about twice a week. i expect them to start producing tomatoes within about two-three weeks (middle to end of June). I hope this helps.