Before I get started, to keep things black & white.... I'm GUESSING your 4 hours isn't actually 4 hours. What you mean is 16 MAN hours. Now I could be wrong... like I said... I was just guessing. That could get you in some hot water down the line if you don't refer to things as man hours. And if you don't know what man hours means, just ask.
You will probably have the same problem as me.... that $18-$25 an hour commercial cleaners charge just doesn't work for me. lol The way I bid is, take a tour of the property, as we are walking I can figure in my head how long it will take to clean. If I think it will take 3 man hours then I x our hourly rate = $total. Go at your own pace a few jobs and compare to the rate charts and if it works out, go by the chart. If not, adjust it to your production rate.
Plus, we take less man hours than most residential cleaners because we only send 1 cleaner in which in turns makes the man hours less which makes the total price less.