Took our daughter there last summer.
She's 6, and tall for her age and there were STILL rides she couldn't go on, and MOST rides she didn't want to go on.
If you take a 4 year old there, you might get him/her on the carousel and a couple of the tot-rides behind the castle, It's a Small World, and that's it. The rest of the time you will be standing in line waiting to get a hugs from Buzz Lightyear and some guy in a Rat Costume. The one and only time we saw Mickey or any of the classic characters was 30 feet up on a stage or a parade float.
And yes, There's big signs everywhere that say people with health conditions including pregnancy shouldn't ride rides.
Funny thing:
We went in Mid-July and the place was PACKED. There was a 5 hour wait to ride the recently renamed Nemo's Adventure (formerly known as Captain Nemo's Submarine Voyage) They now have a fast pass thing where you get a slip and it gives you a time to come back. When you come back you go straight to the front of the line and get right on. I recommend you do this and hit Fast Passes for every major ride first thing when you get there, then in between rides, go do the less popular attractions, like The Tiki Room. (Classic 50's Disney)
Another thing you'll notice is that most rides have been re-titled. Like Swiss Family Robinson's Tree House Is now Tarzan's Tree House, and Pirates of the Carribbean is now "25 Minutes of Nauseating Johnny Depp Fantasy."
Another thing is that all the rides were breaking down left and right. Thunder Mountain shutdown 1/2 way though and we got to walk down the maintenece path to the emergency exit. Disneyland is a very filthy place where the public isn't supposed to see. All the surfaces are caked with decades & generations of chewing gum that's been spat out. Not to mention all the vomit, trash, and who know what else. You just cant see it because it's in a darkened tunnel. Space Mountain-- when we came in at the end to get off, the car suddenly sped up instead of slowing down and went flying past the attendants and then slammed to a stop 20 feet down the track. All the lights flew on, flashing red emergency lights, we could see all the operators behind their hidden windows scrambling around with computers, levers and buttons shutting down the ride. It took em about 40 minutes to get us out of there. The Haunted House broke down on us too, but it was my wife taking our daughter through for her 3rd trip. Another ride that quit was that fake steam train that circles the park. That made for alot of walking to get around. All that in one day.
There. I hope you know what to expect now.