The pitch of a note on a plucked string depends on the tension of the string, the length of the string, and the mass of the string. Think about just the open strings for a moment. So, to get 4 different strings to play 4 different notes while having similar tensions, the only thing left to vary is the mass, which is the thickness. If you wanted to have a thinner string with the same length sound a lower note, you lower the tension. The problem is that when the tension varies significantly, the character of the sound (the timbre, the overtone series, etc.) starts to change. You can certainly try, but I think most people would agree that at a certain point, they no longer care for the sound. I tried to use a fairly thick classical guitar nylon string (plain, not wound) as a low G on a tenor. Although the string was thicker than any of the others, I always thought it sounded horrible. Intonation was bad and that one string's sound did not blend well with the others. I swapped it for a wound nylon string from a similar classical guitar set, and it sounds great. The windings add mass while keeping the flexibility of the nylon core. This way, you get lower notes with similar tension and thus, similar timbre.