LED turn signals are great - they plug straight in, use less power, and create less heat. They're also more noticeable coz they don't "fade" on and off, they have a more distinct flash. They use less power because they have a much lower "load", however that tricks the standard flasher into thinking a bulb's burned out so it flashes at a faster rate to warn you.
Load resistors add that extra "load" back into the circuit. There's no doubt that they bring the flash rate back where it should be, but they require messing with your wiring and remove the main benefit of having the LEDs in the first place! Using a dedicated LED flasher is really the preferable, plug'n'play way to go.
If you absolutley MUST use load resistors you don't need four - having one on the left and one on the right'll be fine. In fact you could get away with just ONE of them between the flasher unit's output and the bike's earth. Best place I've found to put em's on the right side of the bike near the side cover, attached to the frame to help shed heat. You can also use the turn signal wires in the alarm connector, assuming you don't have an alarm (or even if you DO have an alarm, if you're careful). You're probably better off getting commercial load resistors than trying to find appropriate valued ceramic resistors, as they usually come in a diecast, waterproof case that has mounts etc to assist in shedding heat and helps protect the ceramic resistors. If you go the way of making them yourself don't try using carbon/metal film etc resistors, they won't handle the power requirements. You'll need ceramic ones, probably 10W or better.
