Hi S., you lucky mother! You are truly blessed with two children who sleep. Really, what "should" be happening doesn't matter in the big picture, each kid is different. "Sleeping through the night" is defined as 5 hours+ in what I've read, and in fact, your younger child IS awesome and a blessing! You really have been lucky with both kids.
My son, now 4 3/4 months old, was a great sleeper according to me, waking a few times a night (2-4) to eat and then went right back to sleep, until he turned three months old. Then something happened and he has been waking 3-7 times a night to eat or just wake up and at least once a night needs more than 10 minutes to go back to sleep. His napping is our focus, hoping that a good napper means a good sleeper, but we are still trying. He is 19lb and 27", a very healthy boy, and seldom cries.
I am not a fan of "crying it out" at all. I know many are, so I won't diverge into my arguments here, but try the book The No Cry Sleep Solution if you are concerned. It is working for us to get our son to go back to sleep on his own. It's just taking time, as any lesson does. However, I don't think you have anything to be concerned about and really should be counting your blessings that you are sleeping more than 4 hours in a row regularly.