Well, you really just asked the same question 3 times here.
You don't start gaining legacy experience until 1 of you characters has unlocked the legacy. So if you level 4 characters to level 30, before finally pushing one past the first chapter and unlocking your legacy, then yes, you will miss out on some legacy experience. Tho, the early levels don't account for much.
As soon as I unlocked my legacy, I let that character sit, and started leveling an alt, I got that alt to level 19. It only built ~15-20% of a legacy level. I switch back to playing the first character, from level 32-44 I gained 3 legacy levels, and am close to pushing the forth.