New to Four Stroke engines, I'm putting a SAITO .91 AAC in a Great Planes RV4 .60 ARF. What would be a good prop for this plane and someone who's just starting into Four Strokes? I'm guessing a prop in the middle of the manufacturers suggested props? Thanks.

Keep in mind that 4S engines do not generally turn as many rpms as their 2 stroke brothers. I have been known to use 7 pitch props on mine to keep the speed up. For example on my Magnum 52 I have found that a Master Airscrew 11X7.5 is an excellent combo (thanks to my buddy Joe for that advise).
Also know that not all props are created equal(3series, K series, APC, Top Flite wood, etc etc) your selection will depend on your experiences.
A look at the Top Flite size chart suggests a 15X8, I would maybe try a 14X7 personally.
Finally I like to take a hand full of props to the field and fly each one back to back(adjusting the engine to each one) and it gives me a feel for how they each make the plane perform. On my 2S engines I normally end up with a slightly lower pitch than expected , I think I like the throttle response.
Sometimes I end up with a wood prop, sometimes a Master Airscrew Kseries or Scimitar, ya just never know.

Select the prop the plane flies best with, to your taste...

The first to try would be from the range Saito suggested.
Then it is up to you to experiment...

Props are a series of trade-offs. If you compare props of the same quality and manufacturer; one that will increase the model's flying speed, will necessarily reduce its vertical performance; Etc..

I would start with a 14 X 6. That is about the middle of the range. I rarely change from that on a .91. I have used 13 X 8s and a 15 X 4. But I find the 14 X 6 gives me the best performance.

My opinion is that the APC 14x6 or 14x7 will offer you the performance your looking for with that air plane. All the Saito 91's that I have had and still use are not picky on props but I always try to keep them tuned to 9,500 to 9,600 rpm on the ground as they do unload a little in the air.
Fred

On my Funtana I use the recommended 16X4 Kseries prop. It isnt fast but the thrust is excellent.

My Skybolt mounted 91 seems to work best with the MA 13/8 3-blade. This gives good all around performance for my type of flying for this aircraft, but as in the real aviation application, prop selection should be tailored to your own needs. I get decent pull, but have to seriously throttle back when straight and level or the plane looks fast enough to rip the wings off, (definitely NOT scale).

For example, if you intend to do lots of vertical work or hovering I would definitely recommend the lower pitch end of the range. If you want to get scale "Mach" then higher pitch is the way to go. This is true of any prop, regardless of the diameter or number of blades.

Another factor is what diameter you can work with for scale looks and ground clearance. A 3-blade is normally smaller diameter and due to this also seems to have less torque effects at low-speed, high-power flight regimes. On the 'Bolt, if I try a larger diameter 2-blade, the plane can have better vertical performance, but the torque effects are much more pronounced.

4-strokes are noted for their torque and lower top ends and the 91 is no exception. Whatever prop you select you should tach to ensure you don't push the motor much beyond the mid 9,000 RPM range. Any higher and along with increased and unnecessary wear you run the risk of valvetrain float.

That higher torque allows for a larger diameter prop. This is great in scale apps as you can completely enclose the engine while having a good amount of the prop extending beyond the cowl for good thrust. I use this for my scale radial installations.

You'll love Saito. If properly cared for they should give years of great performance. My 91 has well over 330 flights on the 'Bolt, which equates to 75+ continuous HOURS! To note, this is the early version with non-repairable valve seats.

The only issue encountered was one I caused myself. On the first cowled flight the engine overheated and siezed, damaging the conrod big end. I polished it out at work and promptly put another 250 flights on it before it finally gave up the ghost. A $15 replacement rod was all that was required to return to service, but I opted to do a complete teardown and bearing replacement.

Since then I have remained a Saito fan with many scale apps running their motors. Have a few still in storage waiting for the right model. You can tell when a manufacturer opts to only make 4-strokes they will definitely work to make them the best. If I have any complaint it would be their prop selection guide tends to be a bit too aggressive. The larger and higher pitch end of their scale seems to lug the motors down too much.

p.s To solve the overheating on the 'Bolt I followed the same rules we use in the aircraft trade and with proper ducting the motor never overheated again. I followed these techniques in any subsequent installations with the same great results.