4 blade prop ... pitch?

I have a 21' I/O with a Chev 305/Alpha One drive. Currently running a 3 blade 19P and turn about 4200 RPM WOT at a bit over 42 MPH, but I think I need to back out the secondary spring a bit as I'm not sure secondaries are opening fully (Quadrajet). Considering going down a bit in pitch as around 18-20 MPH (pitot, so who knows waht it really is) it takes some work to keep a steady speed on plane. Was thinking about trying a 4 blade since a lot of folks say they give a better holeshot and hold plane at lower speeds (my holeshot is fine my needs, looking to make holding speed easier for my driver).

So does anyone run a 4 blade prop on an I/O? What do you think compared to a 3 blade? How much pitch should I go down when going to a 4 blade?

4 blade 17 pitch. If possible, the 5-blade also helps. Same 17 pitch.

I was where you were at 2 years ago. Boat came with a factory 21pitch 3 blade aluminum. I swapped down to a 19 3blade alum. I then swapped to an 18 pitch 4 blade. Night and day difference. Holeshot was great, really torquey, and speed control just got better. Swapped up again to an 18pitch Stainless Steel 4 blade. Havent tried it yet, but I couldnt imagine it got worse. From what I was told an 18 pitch 4 blade is the same speed and WOT limits as a 19 pitch 3 blade. 17 pitch loses alot of top end. the 18 pitch planed our boat quick and gets on plane around 18 and is on plane from there out.
I have my 18 pitch alum 4 blade (Michigan Wheel Vortex) for sale if you may be interested. Its off my Tahoe Q6 with a 4.3 merc.
Ill sell it with the XHS interchangeable hub too. let me know. Bought it off Iboats about 3 months ago for 140. The black paint has wore off the prop in spots, but it works fine.

The 4 blade Mercury Vensura prop will do the trick. It's got a lot of surface area for hole shot and you can adjust the vents to keep the RPM up when you are starting. You've got extra RPM at WOT so dropping pitch some won't even hurt the top end. 17" or 18" pitch will do the trick. I'd agree with Karl that the 17" might be best. I'd go with a Stainless prop. The thinner material is more efficient. Dropping the pitch, and thus raising the RPM at towing speed will give you better control, as will changing from a 3 to a 4 or 5 blade.
One thing you might want to have checked is what your current prop is for sure. I've seen several where the rated pitch was not the actual pitch. That applies to the new prop, too.

Thanks for the tips and experience. I found some good prices at propmd.com for the Michigan Wheel props on sale...

http://www.propmd.com/pc-737-346-vortex-4-blade-aluminum-propeller.aspx

They have SS Michigan Wheel for what looks like a good price too. A bit more than I'd like to spend though. I hate to spend almost $300 and need to change to another pitch or something.
http://www.propmd.com/c-346-alpha-one-86-newer.aspx

In any event it sounds like 4 blade is the way to go.

Cory, that Vortex 4 blade is the same thing I swapped out from. It pulled great and was very consistent. The only thing ive noticed about the Michigan wheel paint is that it has a tendency to just kinda flake off like mine did. If you are interested in trying the 18 pitch out, it comes with the XHS hub kit with it...I'll sell it for 80...Just let me know.
I just bought a Michigan Wheel Apollo 18 pitch and I would warn on propmd...thats who I got mine through...I called him and he said he had one thing, called me back the next day saying he didn't have it and couldn't give me the price, so I said ok, get me this prop and he cut 10 bucks off, and then shipped and invoiced me for the wrong pitch prop that we originally talked about. I got that prop friday and my prop on Monday after I pointed out to him he sent the wrong one. He sent me the return label, it was just discouraging that he didn't rush ship the other since it was a mistake on his part. Anyways, just call and make sure the invoice is right for him...good guy, just maybe gets too ahead of himself.