1. This phrase is a quote from Psalms 133. It means something like "how nice and lovely". It is used almost exclusively with the rest of the original sentence: הנה מה טוב ומה נעים שבת אחים גם יחד. (Lit: How nice and lovely it is, when brothers are sitting together). The biblical interpretaion is: look how good it is, when the Israeli tribes are living next to each other and cooperate. Nowadays we use this sentence as a popular children's song, usually around a campfire or other social events of that kind.
2. על הכיפאק is a slang experssion, derived from Arabic. You can use it as a substitue to 'great' 'cool','fun' etc.
טדי הוא בחור על הכיפאק (Teddy is a great guy)
איך הייתה החופשה? על הכיפאק! (how was the vacation? fun!)
3. על החיים used mainly with its counterpart: על החיים ועל המוות (Lit: [i'm gambling] on life and death). It's pretty much the same as "Here goes nothing", being said before one submits a risky action.
4. להית' is an unformal abbreviation for להתראות.
אז להית' = then bye.