all stars music is a great hotel... they have a terrific food court... pools if the weather permits... your 4yo will enjoy it... as will you!
if it were me, i think i'd try to hit magic kingdom (hopefully you are early risers, 'cause getting there early is a must-do) and maybe animal kingdom... call 1-407-wdw-dine to make a priority seating for a character meal if you can swing it (breakfast starts at about $20/adult & $10/child), depending on what characters you/child like (suggestions: winnie the pooh & friends = crystal palace at the magic kingdom... they serve breakfast/lunch/dinner. minnie/goofy/pluto = liberty tree tavern @ magic kingdom for dinner only. donald duck, mickey, pluto, goofy = donald's breakfastasaurous at the animal kingdom, breakfast only. there are other character meals but they are mostly at the resorts, which can be tricky getting to, but you don't need a ticket like you do for a park, so it may be worth it! our family's fave has always been a late breakfast at the grand floridian hotel w/mary poppins, alice in wonderland and a few others)
my next best suggestion would be to get your hands on maps of wdw theme parks as soon as you can... get familiar w/the layout... read descriptions of the rides and attractions (at magic kingdom do not miss Mickey's Philharmagic!) so you will know in advance what you want to see/do and what you are comfortable passing by...
for something fun to do, you might want to consider heading to downtown disney and checking out the
lego store w/the playground... my kids loved it (and it's free!) fromthere you can catch a bus to the fort wilderness campground where you will find a petting zoo and some other fun stuff to do (for free!)
epcot also offers some fun kid-stuff... my children have both always loved the journey to imagination with figment (it's near honey i shrunk the audience show)... the world showcase is fun to walk thru and offers some great places to eat (both sit down and counter service... international cuisine)
i suggest you bring your own stroller for the baby... you may want to
rent a stroller for the 4yo depending on how much walking he/she can handle... or if you have a double stroller, perhaps occasionally hitch a ride w/baby!
have you made arrangements to get from the airport to the hotel? if not, you may want to check the transportation board here at the dis where you will find information on town cars, taxis, shuttles, etc... just be aware that even the "cheapest" will cost a few bucks (a taxi is about $40-50 each way).. the transportation board will also give you info on the disney buses... they have a great system, well marked buses and bus stops, very efficient, but in a time of high crowds, you can expect to have to wait a bit...
also, upon check-in, get information from the front desk about shows, parades, fireworks, etc... in order to get a good viewing location, you may have to "stake out" your spot an hour or more in advance... (think st patty's day or thanksgiving day parades in nyc!) only you can decide if it's worth it (for our first trip when the kids were 2 and 4, we found it was better to let the rest of the world go to the parade and we enjoyed toontown w/minimal crowds at that time!)
and speaking of toontown... that is one area your 4yo will really enjoy... mickey and minnie's houses! the judge's tent w/the character meet and greets (BRING A CAMERA!!)... goofy's barnstormer roller coaster (kid friendly!)... and the railroad station... be sure to take a ride on the railroad
last but not least, is disney-mgm studios... a definite mix of adult and kid-friendly... there are some great shows that should not be missed like the little mermaid... also playhouse disney is great for 4yos! if you can get there, you are sure to enjoy those things... but remember, w/only five days in crowded season, you will not be able to do everything! but you will be able to enjoy if you are armed w/a few ideas of what to expect and decide in advance what you want to see/do and what's important to you...
remember, everywhere you go, it will be crowded... get a taste for disney and perhaps visit again at another less-crowded time of year...