Q.How to care for indoor bamboo
I have a bamboo plant indoors that I have had roughly four years. It has only ever been in water, no soil. It is in a case, but as it grows taller it’s starting to fall over. I have no idea what to do with it now.

You may want to move your plant to a pot now, instead of growing it in water.
Choose a pot that is adequate sized, but not to large. Use a good potting soil mix.
Here is a link to help you get started.
https://www.gardeningknowhow.com/houseplants/lucky-bamboo/growing-lucky-bamboo-indoor.htm
https://www.gardeningknowhow.com/houseplants/hpgen/learn-more-about-repotting-houseplants.htm