Each pin of the Arduino must be limited to 40ma maximum current output or you risk burning it out. That won't be enough to move a robotic platform.
Should I use one motor for every wheel or one for the front wheels and one for the rear wheels? I don't to just power one of them.
Would you use a servo to steer or just change the speed of the motors in this case i would have to use different motors for the right and the left wheels.
That's something you'll need to decide for yourself. Using different motors for the right and left side is usually the simplest solution so you don't have to worry about all the linkages to turn the tires with a servo.
Can i power the motors from the 5V outputs of the arduino or do i need an external power source?
The Arduino you're looking at probably only supplies around 800ma or less of current. However, if you're using a motor controller, the motor controller will usually have separate power inputs for the motor power so you can directly connect your batteries that way and not worry about regulation.
Are there any other things I should pay attention to?
Taxes! Always make sure you file on time.