What are the two best parks for a 4 year old boy?

[MinMou]

It definitely depends on the child, and at that age, which characters they like. If your nephew watches the Disney Jr channel, he may have some favorites.
In that case, he might enjoy the Disney Jr Dance Party at HS.
He's still at an age where he probably believes the characters are real, so be sure to bring an "autograph book". (I got a small Disney themed notebook at Target.) I used CharacterCentral.net to find out which characters can be expected, and the daily schedule will let you know where they will be at what time.

 

[a742246]

I'm also a little worried that the MK will close early both days we are there for a Halloween party. (Friday and Saturday.)

If the 2 days that you are going to the parks are Friday and Saturday. I would do MK on Friday and AK or HS on the Saturday. Avoid MK on the weekends if you can.

 

[MudQueen22]

Definitely MK for one park day. The second really depends on his interests. We took our 4-year-old in July. At the time, he was so incredibly interested in all things Toy Story, and also had an emerging interest in Star Wars. He really enjoyed Hollywood Studios. We splurged on the Early Morning Magic (EMM) in Toy Story Land at HS. He enjoyed meeting the characters, and Toy Story Midway Mania never got old. lol He also enjoyed meeting BB8 and watching the storm troopers march through the Star Wars area.

If your nephew is really more into the fabulous five, I would stick with 2 days of MK. If he likes Toy Story or Star Wars A LOT, then give HS a day. If he likes animals A LOT, then give Animal Kingdom a day.
Our son liked Animal Kingdom, but if we only had 2 days, he could have done without it and would have been happy with MK and HS.

I'll also note that the first time we went to WDW as a family, dd was five. We did 3 MK days and one Epcot Day. That was perfect for her. She was very much into princesses.

It will largely depend on the interests of the child.

 

[Azred]

Animal Kingdom and Magic Kingdom

 

[GCorbett]

We took our 2 1/2 year old twins in January of this year. Of course, they are at a different stage, but we found Hollywood Studios to be a bit of a nightmare due to the crowds. They did love the Disney Jr. Dance party, but if we were doing two days, I would choose AK (and MK of course) because it was less hectic. Toy Story Land was basically a mass of people and we were there on what was expected to be a low crowd day. You could hardly navigate no less really see anything. I think the popularity of Star Wars is making the whole park temporarily (I hope) congested.