Hissing definitely is a sign that a step forward was made towards healing.
If the process that used to lead to an intrusive high pitch sound, so neural impulse of high intensity changed into a phenomenon that leads to only a hissing sound, so less intense neural signal, that means that the variables that contribute to the appearnce of tinnitus changed, and they got closer to normal values.
So congratulations and I hope the improvement remains stable or even makes another step forward.
Think about what you'very been doing the past 4 days, maybe you identify "something" that lead to improvement. Maybe you did something or maybe you didn't and there was no exterior factor and the body just healed (went towards normal) inside.
"Some" healing definitely took place, but there in so guarantee that the healing will continue, nor that things can't slip back to where they were.
The same thing happened to me, my T in left went away and the high pitch T in my right ear that is killing me right now had been transformed into a totally manageable slight hiss, but that result went away with the first major stress I encountered after this, so my almost complete healing went back to catastrophic T in a matter of ....hours (minutes, actually)
Say away from stress and stimulants, eat and rest well, exercise, thse are the only things that depend on you with which you can try to "lock" this improvement..