1, 2 and 3 are correct.
4 - I'm not sure what you mean. Are you asking what the person had available to eat as a snack, or are you asking them what they ate as a snack?
Either way, "What did you have for snacks?" would work.
5 - What did you have for breakfast?
Remember that "had" when used in the interrogative becomes "did I/you/he/we/you/they have". You need the auxiliary "did".