I have a build that is 90 grams and runs great with 7600Kv motors. My question is, if everything stays the same and I put on 1204 4500Kv motors on it, would I get more flight time? More power or torque? Or both? Thanks
Lower kV motors will spin more slowly but with more torque. Spinning more slowly (with the same prop) will give you less thrust and less power (and thus lower current) at full throttle.
However, a well setup quad doesn't spend much time at full power, especially when you're considering your total flight time. The amount of thrust required to fly is still the same because the weight and drag haven't changed, so if the props are the same they will still need to spin at the same speed. To reach that speed, you'll need more throttle. Top speed will be reduced and if that leaves you flying more slowly than before, your flight times will increase - but you could probably get the same effect just by flying more slowly.
There may be some difference due to the relative efficiencies of the two motors, but it will be small. I don't know which would be better and you probably won't notice it.
However, that assumes you keep the same props. A lower kV motor really needs larger props, so it can generate the same thrust at lower RPM. Larger props should be more efficient because they act on more air and don't have to move it as fast. That should give you longer flight times and more thrust at low speed, with a possible reduction in top speed.
Of course, all that is irrelevant if your frame doesn't have space for larger props.