I have a honda four stroke on my weed whacker.. The RPM's don't climb like a two stroke, and for the application of a trimmer the engine is not well suited for it. Quiet, sips gas, no mixing, but the power is better down low.. I purchased a two stroke trimmer last year and it functions much better.. Power is there up at higher rpms which is better suited to trimming.
If you need power in the lower rpms, get a 4 stroke. Higher rpms power? Two stroke all day.
Tillers have traditionally used 4 stroke motors, and I would be inclined to stick with it.