A fence I share with my neighbor is falling down. I live on a slope, so a retaining wall is needed because their property is about four feet higher than mine. I've met with a fence specialist, who says the retaining wall should be wood. I've met with a concrete specialist, who says the retaining wall should be concrete. Who can I meet with who will tell me what the best solution is? I've looked around the neighborhood and my neighbors seem pretty split on the types.
You actually want a retaining wall expert. Wood and concrete are both valid materials to use, as long as the wall is well built - but I'd expect block or concrete to last longer than wood. Possibly the person who specializes in building retaining walls would also be interested in building the fence, or you could hire it out to a fence builder. At least, tell the person designing the retaining wall where the fence is planned, and they can advise how to best anchor the posts without weakening the wall.
A four foot retaining wall is starting to get into the territory of where it should be an engineered structure. I wouldn't trust someone with the job unless they specialize in retaining walls, or were following plans drawn by an engineer. Dirt is heavy, and retaining walls have to hold a lot of weight. If you have structures or other heavy things built near the top of the wall, that also imposes an additional load. It's easy for a novice to under estimate how strong the wall needs to be.