We travel via plane at least 4-5 times a year. All our flights are 4 hours or more. He is four right now so about the same age. Here is what I do ...
Take a bag just for him. I let him choose what toys he would like to bring on the plane and then we narrow it down with my input being the final.
I always screen for the "noise" factor, also.
Typically he decides to bring a couple rescue heroes as well as three or four of his Buzz Lightyear little characters. He also has some small metal airplanes (the size of hot wheels) and a couple small hot wheels cars. All these things are pretty small and don't take up a lot of room. I keep them separate by using freezer weight zip loc bags. He can't play with one until the one he is playing with is put back in the ziploc and then in the bag. Helps us keep things in one place. I also bring along a small magna doodle board. On this we practice writing his words and numbers. Works great.
In addition, I select 5-7 lightweight (paper) books. He really likes Clifford, Franklin and that type of thing. On take off and landings...our routine is to read the books. He drinks his juice (which I put in a tippy cup - so that he has to suck on it as the plane goes up - helps with the ear pressure) and I read books to him. On landing we do the same thing but as the plane touches down we practice counting how long it takes for the wheels to hit the runway.
Finally, I bring several different coloring books and colored pencils (they aren't as messy as crayons) as well as preschool writing/number books with mazes, etc. Keeps him occupied for awhile.
While I know this sounds like a lot what I found was that he would spend about 10-15 minutes with one thing and then was ready to move on to the next thing. We recycled toys pretty often but it keeps him busy and quiet. One of my biggest concerns is that he not disturbed other passengers around him by crying, kicking the seat, bumping into them, etc. I found that having these things helps. And, they all fit in his bag along with his blanket, blue bear and of course, snacks. He seldom likes what they have on the plane.
Having said all that, the last trip I brought along our laptop computer with earphones. I also brought a couple of his favorite computer games and a movie. This worked great. He played that for about 2 hours (we have 2 batteries). And, to top things off, I brought his GameBoy Advance (with earphones) and he loved playing that. Kept him quite busy. I know this is the easy way out but whatever works - within reason - is fine by me.
Hope this helps. Let me know if you have any other questions.
One more thing. Plastic zip loc bags - freezer weight - work great for trash, etc. while on the plane. I always carry about 10 of them when traveling. They are wonderful for many, many things...especially with kids.