I'm going to be honest. Peter Pan was a disappointment (mainly cause it was talked up so much and the long waits) to me, I thought The Little Mermaid was better and the rides are similar enough that if you are short on time you wouldn't miss much by doing one and not the other. Might depend on if your 4 year old is a boy or girl

If you are going after the Mine Train opens I am sure that will be a must. If your 4 year old is a little girl Enchanted Tales with Belle is wonderful. We all liked Mickey's Philhar Magic. Buzz Lightyear was fun. I can't recall if Barnstormers had a height limit, if the kids aren't tall enough you probably won't want to do that one alone. Space Mountain would be fine as a single rider if the wait isn't long (it really wasn't the day we were there, we went twice cause the wait was only 20 minutes after we did all our 'musts' in Tomorrowland). The cars fit 3 and are single file, since a lot of people go in pairs or groups of 4 it seemed they were looking for single riders to come ahead in line to fill the spots so your wait might be even shorter than the posted wait. Big Thunder Mountain was very fun, one of our favorites but the waits were long. You'd want a fastpass for it (maybe you could get a fastpass for that at the same time your family gets a fastpass for a character meet) I didn't see any single riders but I don't think you'd have a problem. Splash Mountain wasn't open when we were there (so maybe that made the Thunder Mountain waits longer?) I got the Passporters book before and liked looking at the rides but I have a hard time making set plans so I just made a list of what rides we really wanted to do in each 'land'. We did 2 days in MK, (one day was Fronteirland, Adventureland and Liberty Square; the next day we did Fantasyland and Tomorrowland) but I bet if you are okay not seeing everything and pick the things you really want to do you can fit it all in.