With a just 4 year-old, definitely fit in
Legoland! We have annual passes there and it is fabulous for kids under about 10...easy to get around property, not too hilly, lots and lots for young children, and still very interesting for adults. Roughly an hour from DL to LL, though do plan to avoid travel before about 9 AM and between 4-7 PM. In making your plans, take note of the fact LL is closed Mon & Tues (I think that's right!).
The Holiday Inn Van Nuys will be fine; I think that's where my dh and I stayed on our wedding night, before leaving on our honeymoon. I will admit I don't remember much about it

It is not a particularly scenic area of L.A., but since you don't sound like you are planning on spending lots of time hanging out at the hotel that doesn't really matter. Please be aware that freeways in L.A. range mostly from slow to slower, especially the 405, 134, 5, and 101 (the ones you are most likely to use!). As with going to Legoland, try to avoid travel between 6-9 AM and 3-7 PM; if you do that, you will do a lot better. Also, take advantage of carpool lanes when you can (2+ people). I mention this so you won't think "30 miles on the freeway...that should be half an hour." It might be, but it also might be more like 45-60 minutes even at non-rush hour.
You'll be very happy with Burbank Airport; it is my #1 choice of L.A. airports. Small and easy to negotiate. For your return flight, take snacks in with you, as there's not much to eat and it is very expensive; with a young child, these things matter!
Look carefully at the height restrictions on Magic Mountain rides for your son; it really is a theme park best suited to older kids and teens, with lots of big roller coasters that have minimum height requirements. My older son first went at age 7 and I considered that a little young to really enjoy it.
Despite its reputation, Hollywood--especially with a young child--is not all that big a deal. There are a few things to see and that's about it. You might consider going to a Metro Red Line station and taking that to Hollywood; then the trip itself is part of the adventure and you won't have to worry about parking. Here's the link to the Metro website:
http://www.mta.net/riding_metro/default.htm
From where you are staying, if you want a quieter afternoon, you might consider the Getty Museum, which is fairly close by.
http://www.getty.edu/visit/ It has a fabulous view, a neat tram ride from the parking to the site (museum is free, parking is about $5), lots of space for running around outside and uncrowded galleries, and a good family center that helps make the whole trip fun for kids. Too many days of theme parks in a row can be exhausting for everyone! The beach would also be another good place for a low key day. Also close by and free is Travel Town in Griffith Park...lots of old trains to climb on, and a favorite with both my sons.