At a stop, four fingers is probably appropriate. Each shift thereafter, the clutch doesn't need to be pulled all of the way to the bars and, in fact, tends to shift nicer when just pulled enough to unlock the clutch but probably still dragging a little. That may sound wierd but when you get consistant "no clunk" shifts mastered, you'll find that's what you're doing in conjunction with timing the throttle position and speed of shift. Many will suggest and practice that two fingers is the best technique for that, and I wouldn't disagree. A standard clutch adjustment with just a small amount of free play when fully extended should be fine for that. If yours is hydraulic, you don't have that option anyway.
Now, that being said, I'm pretty sure that I almost always use all four out of habit. The bikes I started on required both hands sometimes.
I never gave it thought until engaged in a discussion a few years ago. I realized I didn't know so paid attention and found it was all four I was using. What you are feeling may be an indication of a different problem, that of anchoring to the bike through the bars, something you should ideally never do. On hard braking, a time that may coincide with downshifting, it's likely that you are still putting weight into the bars but shouldn't be a problem because you can take it through your palms, not requiring a closed grip to work. I have tried two finger clutching but found it to be clumsy, probably because I've done it with four so long, and didn't find any advantage to changing or re-learning the other, so I didn't. If you get used to two, you may be closer to the norm for track riders but I'm not sure it's a big deal either way. On the other hand, light on the bars, other than initiating turns, IS a big deal. Read up and work on that, for sure. I feel that's more important to have right than how many fingers on either lever.
Now, for real confusion, I find I'm more likely to use two fingers on the front brake, the one thing where you might think the additional force of all four would be helpful. I definitely used to use all four and find I still do on hard, straight up stops on the street, switching between the two reflexively. Some of my older bikes almost required all four due to having less effective brakes. The one I have now I consider adequate but many don't. The weight alone is 608# and the rotors aren't as big as most of the newer bikes, therefore slightly less effective.
The reason that I switched to two was, when I started to learn to trail brake into a corner, I needed the smooth control and ready smooth transition from brake trailing off to throttle coming on. Even though my bike doesn't have all of the brake it might, it's still sufficient to lift the rear tire on hard braking at the end of a straightaway. My hands are pretty strong and I realize that I'm squeezing AWFULLY hard sometimes with just two fingers at the ends of the big speed straightaways at the track, but it's enough and the control is necessary if braking past the entry. I also have found that I will move my hand out further towards the bar end to increase the leverage on the brake lever, helping with that. Those were things that were done unconsciously but I noticed I was doing.
BTW, IMO, don't worry about trail braking until you have EVERYTHING else about cornering mastered and totally reflexive. Your lines, looking through the corner, appropriate entry speed, braking completed BEFORE to turn in, picking up the throttle smoothly, light on the bars, and totally relaxed. When you're COMPLETELY comfortable with all of that, totally relaxed and hitting all of it reflexively, you'll be a very quick rider and will have a totally relaxed feel entering and coming out of corners STRONG. That will be many, many miles of extreme twisty riding and practicing to accomplish so don't get impatient. When that's ALL been done and it's a consistant, always relaxed way to ride, it's not hard to add trail braking the front in. If you play with that first without the feel of grip and accurate gauging of corner speed that starts on the straight before you ever initiate the turn, you're just hurting your learning curve and are likely to hurt yourself. JMO, hope it helps.