Flush Woes

I am completely new to this game and trying to teach it to a friend (also brand new to the game,) who has a tendency to argue everything and tie things in knots. At least I know what question I need to ask all of you:

Can you count a flush during "pegging" (I did use that term correctly, yes, no? When you are playing alternately?) And if so, how many of the same cards do you need to have for it to count as a flush? And, correct me if I am wrong ... you must have 4 of the same suit to count flush in your hand (five if it's the crib hand.)