4 to 1 is actually pretty good. Mine is more like 6 to 1, and I am using a 2" cylinder. If you need more grunt, add a second cylinder - doesn't look like it would be too hard.
I was going to build an auger too, but they can be tricky. There is a guy on here (renopker) who is building one right now, but he was lucky enough to score a sweet deal on a planetary reduction - and he has a machine shop. I did some research and calculations on a chain reduction drive unit (I would have sooner had a gear reduction), but then found a reasonable deal on a brand new one (bevel gear reduction), so I went with that (if interested, Google Farm Fleet - page 127 in their catalog). I had a big post setting job that I needed to get done, so impatience played a part here, too!! It will eventually pay for itself, but I will have to set a few more posts.
If you can find a 3 point hitch, PTO drive digger on the cheap, this may be your best bet on a home build. It may take some doing to adapt a hydraulic motor, but should give you good results in the end. Surplus Centre has great deals on hydraulic motors, you will need to do some math to size the motor to your machines aux. flow. Most PTO-drive augers have about a 3:1 reduction ratio, so size the motor to turn about 150-180 RPM tops. It is better to turn the auger slower than faster. Mine turns at about 30-40 RPM, which I like.
Happy Building!!
SR