You don't HAVE to transplant... you can start the seeds in the final pot but, as UnkleSkunk said, you'll struggle in the beginning. And there's also no set size of pot nor is there any set progression of sizes - it's what you have and what you're comfortable with... When you've got a really good rootball, it will be time to transplant and the new pot should be at least twice the size of the old pot - but it can also be larger. Again - it's what you have and what makes you comfortable but if you've started out in a 4L pot, I would suggest going straight to the 18L ...
Good luck