Taking time off from a brand new job to interview for a better one

[Bennie]

I've very recently started a new job after almost a year of looking. It's a fine job, an upper management position and one I can do well although it's not challenging and has little growth potential. On my third day on the new job, I received a call to interview for a job I had applied for several months ago. When I returned the call, I realized I was in an interview on the spot! At the end of the interview, the hiring manager said her assistant would call me early this week to set up an in person interview. Great! EXCEPT... the interview is one of two on-site interviews (if I get past the first one) and the site is four hours away--at best I could put in two hours at my current position before leaving for the day to catch the plane. Advice on how to handle this at the new job without tipping my hand? Should I tell the new potential hirer my problem? I want to be ethical and I also really want this other job.