AHi all , I am having my first bake sale this Friday and will need to transport 50 cupcakes to another state . My question is , will buttercream frosting withstand 4 hours of car ride ? If I add milk into the buttercream frosting , will it turn bad ?

You don't say where you are located, but in hot weather you can keep your car's air conditioning cranked and the buttercream should be just fine.   Also, with most buttercream recipes ~ the high sugar content will keep dairy products from going rancid too quickly.  

If you are making it with all butter be carefull if you are using Crisco or half butter and half Crisco you should be fime. Maybe box them and use some ice bags. good luck

AAnd keep in the shade. Even if the car is cold, the sunlight hitting the cupcakes will melt almost any frosting, except maybe fondant.

AThank you all for being so helpful ! :))

AWhy don't you pack the frosting into a bunch of piping bags, put in a cooler, then frost when you get there? Leave a bit early.

AMy next question is , how fresh do cupcakes last ? If I need it by Saturday afternoon and bake them on Thursday night , will they still be fresh ? :) thanks !

they should be fine for about 3 days total - especially if you box them, then put the b ox into a lg plastic bag.  Secret Agent offered a good suggestion.