Ok I have done much search and reading SO PLEASE don't tell me to "search." I did that but I'm looking for personal experiences!

I recently have started a job where I can bring my bike when I travel and commute from hotel to job. Very excited!

One job is 4 miles from my house. I'd just ride my regular bike but there is no safe place to lock it up. So..
4 miles, with hills. Would like to be able to fold easily (points for Brompton). However folding super small isn't that important but I WILL be carrying up a flight of stairs or two.
I guess Brompton would solve these problems but I do like 20 inch wheels and really hate the idea of dropping $2000 on a folding bike. How cumbersome is a Zizzo or Dahon to carry around? Any other suggestions? Those are the bikes that seem to come up the most.