Tense is variable to any perspective of writing. You could use future tense in first person, for example. Tense is time, where person is perspective. I don't think future tense is rarely explored either - we use it all the time, but not on it's own because it doesn't express immediacy properly, even though it is unavoidably immediate (hence it's kind of a self-contradiction).Expressing immediacy well is essential to first, second or third person writing. If tense is variable to all of these perspectives, then I think that we would be going down the wrong way if we were to consider it as the key element to fourth person. Writing in future tense wouldn't change what "person" you are writing in, I think it would just make the writing more rigid. I think that fourth person needs to be a whole perspective, not a tense. Simply making a weird narrative style doesn't mean you've created fourth person, lolz.