We are talking Sans. here. 4.5 feet Wow. Don't trim them.
they are not top heavy I don't think, planting in bigger pots will not eliminate plants falling over. Use any fast draining cactus mix. Light sun, they can burn. Yep, you can separate them into more than one pot, but they like to be crowded. They will all need an 8" to 10" pot. I have a S. masoniana that breaks out of the pot every three years. So I must divide it.

Cut down on watering and perhaps some more low light, or light shade. Sounds to me you are doing great. Make a few minor changes only and watch carefully what they show.

Some species just grow that tall, expecailly if given enough heat. You must have one that loves it where you live. Do you have any idea of what Sp. it is?