I've been doing cupcake towers for years but I always set up my display 1-2 hours before the event. I want to be able to set them out about 3-4 hours before the event without them drying out. Any secrets? I've heard of putting vanilla syrup on them before icing - but I'm still concerned about the icing hardening up. Help....

Your actual cupcake sponge shouldn't dry out in such a short time. If they are, you have to get a new recipe fast. In terms of your frosting, try one that does not crust, usually by increasing the amount of fat, or a different one entirely like a meringue based buttercream which I believe never crusts (I've never tried them). Maybe someone else can help you further.

Try brushing your unfrosted cupcakes with simple syrup it will help your cupcakes moist for days.... If you need a recipe for simple syrup pls don't hesitate tolet me know... Hope that helps

If you are using a crusting buttercream, it will begin to crust within minutes of being iced. So, regardless of when you set up your display, your icing will be "hard". However, as Dayti said, use a non-crusting buttercream. Both IMBC and SMBC are non-crusting and taste great! You won't have any worries with them. And I would be surprised if your cupcakes dried out in that time too. However, a simple syrup with a flavour that compliments your cake flavour would be an option before icing them.