I have a 4in1 Brushless ESC that only one ESC burned out on, but the rest should still be usable. Is there a way to salvage it and use a single ESC to replace the burned one or something? If so, how would I go about doing that?
As ifconfig pointed out, the most common way to continue using a not entirely functional 4-in-1 is to solder a standalone ESC to substitute for the damaged one(s). Balance is not that much of an issue (that's what the I term is for), unless you're doing the highest performance flying, in which case you should just replace the whole thing.
Also, in order to not lose current sensing on the new ESC, it can pay to solder your new ESC's power leads directly to the VBAT power traces after the shunt resistor on the 4-in-1 (they are usually exposed and sometimes even have metal rails soldered to them). If you don't want to bother or aren't that good at soldering, you can usually make do without rewiring the extra ESC, and just adjust the current sensor multiplier. This isn't going to give you as precise a reading, but it's an okay approximation for gauging general current usage and deciding when to land, just don't rely on it too much. Multiply the number by 3/4 (or less, depending on how many surviving ESCs you have on the same board) for a first estimate, and maybe later make a finer adjustment by measuring the true mAh used from the pack vs. used mAh indicated in the OSD.
There are two more interesting ways to make use of a damaged 4-in-1, however.
The first one is similar to what ifconfig is suggesting, but instead of adding a standalone ESC, you add another 4-in-1 to the same stack, and wire it to the same flight controller (but to different motor pins, of course). For this you can even use another damaged ESC, as not all of the individual ESCs on the board may be used. This also gets rid of the balance problem if it bothers you somehow, and also lets you avoid clipping the wires of one of your motors shorter in order to accomodate the individual ESC.
The same caveat applies about current sensors as with an individual ESC, and it's going to be harder to wire the two power rails together, so you're probably stuck with using just one of the current sensors and adjusting the multiplier.
The second, and more advanced, is to just replace the damaged component on your ESC. It is usually one (or several) of the power transistors (FETs) of the ESC, and those can be easily replaced if you know how to use a heat gun. You'll need the replacements however. Most FETs used in popular 4-in-1s can be ordered, so it's a question of identifying the part number.
You would have to be careful so as not to lose some functionality like current sensing or affect the CG of your quadcopter.
One way I could see this working is leaving the 4-in-1 ESC on the stack in your quadcopter frame and only wiring up one ESC, with the signal wire from the standalone ESC wired to the pad on the 4-in-1 ESC where the dead channel's signal comes in from the flight controller.
In order to not affect the center of gravity of your quadcopter and destabilize it, you would likely need to mount this extra ESC in the middle of your frame. Normally with either a 4-in-1 or standalone set of ESCs, the symmetry the ESC components on the frame balances out and you wouldn't have to worry about messing with the CG too much.
You would also have to make sure that the 4-in-1 and standalone ESC operate on the same signaling protocol (Oneshot, Multishot, DShot, etc.), because Betaflight (et. al) expect all ESCs to be capable of the same signaling method.