It depends a lot on their individual interests. When my older daughter was little, she was afraid of dark rides. She loved princesses. She refused to ride Splash Mountain because the hill scared her.
My younger daughter loves thrill rides and gets bored with smaller rides (like Barnstormer). She loved Splash Mountain when she was three. Can't wait to ride EE, but she is not yet tall enough.
That being said, here is what I would recommend:
MK
SDMT
Splash Mountain
Big Thunder Mountain
If they don't like thrill rides, then:
SDMT (very small coaster)
Talking Mickey or meet the princesses (whichever characters the girls like more)
Peter Pan (Peter Pan is very popular and always has a long wait, but it is not as good as BTMR, in my opinion).
AK
Navi River Journey (unless they love thrills, in which case I would go for FoP- but height requirement is 44 inches, so 4 year old probably not tall enough)
Kilamanjaro Safaris
Kali River Rapids
HS
Toy Story
Star Tours
Tower of Terror (unless they are scared of drops and the dark- my three year loved it) If not ToT, then Frozen Sing Along
Epcot
Frozen
Spaceship Earth
Character Spot (though sometimes the fastpass line is longer than the regular line)