Here is a link to a thread
about this same subject. It may be helpful.
Here are my thoughts. Graphite rods are marvelous things and some will cast a large range of lines. Some rods do seem to have a sweet spot and they only want to cast one certain line. That line may or may not be the line wt that the rod manufacture recommends. Sometimes the manufacture seems to miss the target.
The most important aspect of casting a rod over/under lined is the caster. I believe that over/under lining a rod has lot to do with the ability of the caster to slow or speed up his timing to accommodate the line and rod dynamics. When a rod is underlined the caster has to compensate for the reduce wt of the line out of the tip. So if a 5wt rod cast a 5wt line very well then a 4wt line will have to have more line out of the tip to compensate for the reduce line weight. Saltwater fishers do this all the time. In high wind the saltwater caster may go down a line size to reduce the line diameter with less wind resistance. The saltwater caster then increases the line out of the tip by 10' or so to compensate for the lighter line. The caster must be able to carry the additional line in the air. When you over line a rod you have to compensate for the additional line weight out of the tip by reducing the amount of line out of the tip.
Along with adjusting the amount of line out of the tip when you under/over line you may have to slow or speed up your timing. This is why I think that over/under lining a rod should only be down with accomplished casters with a good deal of experience. An experienced caster can feel the cast and understand what is needed to match the rod action and line weight.
There is one other consideration when lining a rod. The manufacturing of fly rods does not allow the lay up of graphite to be absolutely the same with every rod. Lefty Kreh states in "Lefty's Little Library of Fly Fishing". This is a direct quote from his book: "...I would be willing to bet that if you took 20 rods from any top-line manufacturer, all designated as 6-weight, there would probably be at least one 5-weight and one 7-weight rod in the batch. The rod manufacturers will tell you that". So it is understandable that some people find certain rods work better over/under lined.
So Patrick, getting back to your question about your 4wt. It is not surprising that on very short cast that the 4wt line worked better. If you could have extended more line out of the tip with the 3wt line, it would have cast better. In your situation the 4wt is a better choice but that doesn't mean the rod cant cast a 3wt quite well in certain conditions.