Wiring at 4 ohm rather than 2 ohm should theoretically use 1/2 the current at the same given voltage. However, if the strain is way less on your electrical (not as much voltage dipping) to the point that your able to maintain a higher voltage going into the amp you would use less than 50% of the power. Which will likely be the case in your Yaris.
BUT, I would take the power ratings on that amp with a grain of salt... probably gonna end up being like half of what it's rated for (even half the RMS) and depending on your sub enclosure if you are wiring at 4 ohms your impedance rise is gonna be to like 9 ohms or something playing music most of the time so you won't be seeing whole lot of current draw. Too many variables here, but yes, unless your Yaris has less than a 100 amp alternator you will be okay with that setup at 4 ohms with that amp. I would opt for a good battery though. If it wil fit under the tiny hood. If not put one near the amp, a sealed non-spillable AGM type battery like you would find in a computer battery backup system.
