This application was the only firm I applied to with an extensive amount of words/characters to fill. Although you don't have to use the entire 4000 character cap, I think firms offer this amount for a reason. They want you to fill the space to tell them more about your experiences/personality/interests to get to know you better.
I got past the application stage and used up almost the entire character count for each question. For the hobbies question specifically, I only discussed two hobbies of mine in depth. I think limiting yourself to 1 or 2 will allow you to discuss it more in depth and show your passion for it fully instead of mentioning every hobby or achievement you have!