In other news, I've had to adjust the pH once, but I haven't added any more nutrient solution. The water level has dropped about an inch, but the roots are hanging right down into it and the air bubbler helps spray some bubbles up onto the roots as well.
Here are some new pictures. This first one is a closeup of the plant that had the insect damage and was somewhat stunted. You can see the cut and where the stem split, and look at how much it's grown! It's bigger than the other three now.
Here's a full picture of all four plants:
And it wouldn't be a hydroponic update without a picture of the roots!