i'd try and keep things as simple as possible on your first bot.
2 wheels and a caster is the simplest configuration.
if you really want 4 wheels, i'd stick with 4 motors rather than introducing chains and gears if you want to keep things mechanically simple.
you will be able to use lower power motors this way so there wouldn't be much difference in the power used.
my first bot was a failure because i went with motors that were far too big for the job, which in turn needed huge motor controllers, bigger batteries, etc.
if you want the best chance, start with a nice simple design. the fewer components there are the better. once you have this working you can move on to version2...