Depends: Does your 6 year old like fast rides and if so, are you willing to do a baby switch with another adult, leaving out the 4 year old? Or would you rather do rides that everyone can ride on?
I have a 5 year old and a 2 1/2 year old. Although my daughter likes the fast roller coaster rides, we can't do them when I take both children by myself. On those days, we get to the park at opening and immediately head to Fantasyland. There are quite a number of rides in a compact area that would be suitable for both of your children. Dumbo is a huge favorite of young children. Because it has a limited capacity and loads slowly, the line becomes longer and longer as the day wears on. Go there first right after opening. Peter Pan is another popular ride but also suffers from some of the same problems as Dumbo although to a lesser degree. If the line isn't too long, go there next. Both my children really enjoy It's a Small World as well however at the present time, it's closed for refurbishment. Check to see if it has reopened during your visit.
In Toontown, there is Roger Rabbit's Cartoon Spin. Both my children enjoy this one however some of their friends do not because of the loud noises. You can obtain a Fastpass for this ride if the line seems too long. There's a roller coaster (forget the name) there that would be a good starter coaster for your children if they've never been on one.
If you think your children can handle a couple of fast but short drops, I would suggest The Pirates of the Carribean. Both of my children enjoy this ride. You can also obtain a fastpass for this ride. If you feel the 4 year old is up to it, you might even try the Matterhorn (a roller coaster). It has a low height restriction, I believe 35" or 36". (No, my 2 1/2 year old hasn't been on this one. I still feel he's too young for this ride.)
If you want to go on some of the more popular rides that that your 4 year old might not be able to go on such as Splash Mountain, Thunder Mountain or Space Mountain; then you'll want to approach your day differently.
Some things you might want to know to make the most of your day: You can enter
Disneyland 1/2 hour before offical opening time although only Main Street is open. You could grab a muffin and juice or coffee while waiting for the rest of the park to open. If you are driving, it will take you an additional 20 -30 mins. to travel from the parking garage to the front gates. Factor that into your total travel time.