on a 25 hp outboard are there any advantages of a 4 blade prop versus a 3 blade and aluminum over stainless?

It will depend on the application of the outboard; (is it on a runabout, an inflatable, or a pontoon?)

Aluminum props are the ideal choice for small- to medium-sized boats and motors. Aluminum props are a good choice for use in areas where prop damage is probable. The average aluminum prop costs about 1/2 of the cost of a stainless prop.

Stainless steel props are more expensive, but they offer several advantages. They deliver more performance and are available in larger pitch sizes. One reason stainless props provide more performance than similar pitch aluminum models is because the steel blades are rigid and they don't flex under power. The blades on an aluminum prop actually bend when the prop is in use, reducing the overall size of the prop by approximately one pitch size. Stainless props also provide increased bow lift, especially on heavy boats. The more bow lift a prop provides, the faster and smoother the boat will ride at high speed and in rough water.

The advantages of a 4 blade prop are they provide quicker take-offs, and allow the boat to be kept on plane with fewer engine rpm's. However, they are slower at top-end speeds. A quality three-blade prop is normally two or three miles per hour faster at top end than the same pitch prop featuring multiple blades.

On smaller engines you won't see much differance either way as long as the prop is the right pitch. Four blades work well on higher h/p engines mounted high on the transom, or on jack-plates as there is an extra blade in the water at all times.There also is a growing trend of using a four blade with 1" less pitch on triple engines on the center engine because the outer engines tend to "steal" or starve the center engine from getting clean water,and the four blade provides more "bite".As far as S/S verses aluminum,I tend to prefer S/S because of the durability factor.You will need to consider what type of water you are running in.If you are prone to running across shoals or in skinny water S/S would be my choice,but I doubt that you will see much differance otherwise.Maybe a little better hole-shot,but we are talking about a 25H/P. Don't expect too much.

Marginal difference - you can drop a pitch with a 4 blade and get the same top speed and pickup a little hole-shot speed. SS is good but heavy and wears the dog-clutches a little more when going in and out of gear also if you hit a solid object the prop is not always the sacrificial part - something in the lower unit takes the beating as compared to an aluminium prob that will bend and break (but they are cheap). SS a little more performance but negligible especially on 25hp.

stainless steel prop won't bend like aluminum.

yes