Been there and done that several times...
Portable DVD player with headphones...
Hit up the dollar store...I spent about $20 dollars on "new" little toys, stickers, stampers, note pads, etc etc...every so often pull something "new" out of your bag of tricks...plus if one of the toys is lost on the plane or airport it was only a dollar.
Snack, snacks and more snacks...I could pull out individual snack sized packs of fun snacks...sugary one they didn't get a home...
I had sippy cups for them to suck on at take off and landing (instead of gum)...the stewardess was glad to get me a drink to put in them for take off.
And I will admit I even packed the individual dose packs of benedryl...just in case one needed help with taking a nap...I never have given it to them for sleep...but I was prepared, because well there are another 100 plus people on the plane that wouldn't want to hear my child scream for 4 hours.
Take a change of clothes for them and YOU...I had a diaper blow and got on my shirt too...
The 4 year old might like a hand held game of some sort...Leaspster, etc...some resale shops have them and cartridges for super cheap.
Hope this helps...have a great trip!1