I've noticed some providers ask for a deposit up front if your wanting to book a longer engagement...4hrs or longer.  Are these refundable?

I am not sure I would ask for deposit upfront unless international or specific qualifications.
But you know what these days you never know how people act or things can come up. In the past I have flew to client and kept payment stub to show him, then I was reimbursed with full payment as well.. I am great with extended visits if you show interest!?!!?!  xoxo  Sabrina