If it were me and I had no other choice, I would do 5 pvc (or, wood blades would be somewhat faster) blades (6" pipe)  4.5' diameter (you may wish to trim them down later to get more rpm).

I would make the pvc blades 3" at the widest point, and 1.25" at the tip. (tip being 4 degree angle almost straight up and down)

And You might want to try a "Magnetic de-cogging mechanism" like I did on my treadmill motor mill here:

  http://www.fieldlines.com/story/2007/3/9/2533/99152

Your motor probably has 12 poles and thus 12 big cogs per revolution.  So you would need only 12 little magnets. (guess- 1/2" diam. by 1/8" thick ///and use big pan head wood screws or similar)
The objective with the 12 magnets spaced correctly is to make 24 smaller cogs per revolution instead of 12 big ones.

It is difficult to say, but if you understand the principle well, you might reduce the cogging anywhere from 20-50 percent.

Be very careful to get your alignment exact, or it could make the cogging even worse!
Mine worked quite well for a time, but during a 'hurricane wind' (probably 70mph gust) it screwed the hub on even tighter than I thought possible, and screwed up the alignment an eighth of an inch and worse cogging came back. (I never fixed it with a 'sheet metal washer' because I knew I was going to take it down in the spring and make a bigger mill)

I took that downwind mill down earlier this year since I didn't need it any longer since I have a 6 footer now.

(If anyone ever makes one of those downwind mills, be SURE to put a wood or plexyglass 'beard' mounted below and to the motor in order to help it turn (yaw) better in the lower winds!!)

I sorta like the heavy hub though.  It seems it helps to keep it turning in lower winds better and  keep it from stopping as much from the cogging.  Inotherwords, it seems it smooths over the cogging better in low winds and 'coasts' the prop better.

Wait for a 10-20mph wind day and see what it does.  4 footers don't do anything under 10mph.  My 6 footer only gets 'bits and pieces' of low wind on a typical 5-10mph day which amounts to almost nothing anyway. (maybe 10-50Watt hours??)