I'm building a mini quad and bought a 4s battery not knowing the motors where 3s. The esc's are rated for 4s so I go a 4s battery and relized today the motors are for 2 to 3s, will this work or should I use a 3s battery.
What motor is it? There is some discussion on RCgroups about certain higher end 3S quad motors that run fine on 4s. I would tend to believe that as long as you are not over the safe wattage levels you should be ok. Dig for 4s quad setups and I think you would find that guys are using 3s SunnySky, Cobra, and other quality motors.
I don't know if I'd try the emax 1806's on 4S. There's not much metal in their base to help sink away the heat. The DYS 1806 motors as much as I'm not a fan of them have a much more substantial base that probably helps keep them considerably cooler.
Though I run both on 6x4.5 props with 3S and haven't experienced heat issues (despite quite a few people saying the 1806's can't handle those props) except with the BS12 ESC's.
I have run EMAX 1806 2280KV and DYS 1806 2300KV motors on 4S without issue on EMAX 12A ESCs. The key is to use a docile 5030 HQ prop (less amps/trust than GemFan) to offset the extra RPM and amp draw which results. With the HQ 5030 I could fly the setup as hard as I wanted and wouldn't really risk hurting anything; the extra thump of the 4S made for some great acro flying. I have flown HQ 5040 and carbon fiber 6020 props on these motors, also on 4S, but was very mindful for how long I was hard on the throttle. Since you casual/fun fly with so little throttle (between 30% and 60%), you can get away with what seems on paper like a bad idea. Getting into FPV racing where you carry long periods (> 3 minutes) of high throttle (> 85%) use is a different story.
On my quad, which has motors only rated for 3s, I was getting improved performance on 4s batteries for 5 minutes or so, until one motor on the quad would suddenly stop and it would fall out of the sky in an ugly low-performance way. I checked the temperature on the motors and ESC, neither were particularly overheated. It's a problem I never have on 3s batteries.
I can't explain Zanderlex's issue, but I can say that motors are not limited by voltage (within reason): they are limited by temperature, and accordingly current. It is often the case that higher voltage results in more current draw and therefor higher temps because of the increased RPMs, but if you prop down accordingly and your ESCs are rated for it, then you should have zero problems flying on 4s. Heck, you will probably even see a boost in efficiency over 3s.
Just experiment and check your temps to decide what works for your setup and style of flying!
Like nitrous on a car's stock engine...It will be fun but even if it doesn't immediately blow up. Longevity will suffer.
Especially if you are asking a lot from a brushless motor already. If it is already getting hot from running 3S I would not suggest trying 4S. It can get to a point where the insulating material burns off of the coils. When that happens it will likely burn up the ESC resulting in a spectacular crash.
I used a 4s on a 3s 2212 motor with a 4s rated esc.. after a few stress-tests, it started burning. When the fire was off, I tried to spin the motor manually, but it was really hard to spin. I think, that the higher rpm destroyed the bearing and due to the higher resistance, the amps raised, so it fried because I didn't use fuses. Btw. there was no prop attached to the motor.
I hope, I could help you