My oldest daughter was only 3 weeks old when she first visited the parks. We were visiting close friends to introduce the new family member and, since we were there anyhow, we went to DL and CA, too. This is of course only one mom's opinion, but I don't think either of the above scenarios makes much of a difference, so I would go with what works best for you (convenience of travel dates, price of flights, availability of suitable accommodations, crowd levels, etc.). Regardless if the baby is 4 months or 11 months, you will be juggling a stroller or baby carrier, there'll be a lot of napping in said stroller, and baby will not have any lasting memories later on but will probably be fascinated by all the fancy lights and colors. In either case, there'll be plenty of rides that baby can do with you (though you might cover the little ears in loud dark rides, such as Winnie the Poo).
If you do a front carry with an Ergo, sling, or Bjorn (which I did and loved, since it was much easier to maneuver the crowds, though we in later years appreciated the ability of the stroller to hold much of our stuff), earlier might be better than later, since baby will be less heavy and easier to carry. An advantage of going later might be that DD will be older and probably better to express herself verbally, which in my kids made a big difference in being able to reason and communicate (which can cut down on much stress and tantrums, when over-stimulation or frustration hit). Also, big sis will probably get more lasting memories out of the trip, when she's a bit older. It might also be easier on you to travel with a baby and a 4-year-old than with a baby and a 3-year-old, but that of course also depends on the individual kids' temperaments and personality.
Whatever you decide, make sure to take advantage of ride switching and of the baby center in DLand (SE side of hub, next to Plaza Inn). It was my favorite place (traveling in October in fairly hot weather). Quiet nursing areas, friendly staff, ice cold water fountain, miniature sized toilet for DD, and the opportunity to pick up any baby supplies you might have forgotten. California Adventure has a baby center, too (in the Pacific Wharf area), but I found the DLand one was much more beautiful and better equipped (though that was in 2005, so maybe the CA one has caught up since).
Either way, I'm sure you'll have lots of fun!
