I have very mild weakness in two fingers of one hand and slight clumsimness in the other hand. Brisk reflexes occur in normals, too, but what one doctor calls brisk, another may consider hyperactive. My reflexes were evaluated as brisk, but I believe them to have always been that way. More often, they are normal or reduced in the affected area. MMN does involve atrophy, fasciculations and slowly progressing weakness.
Increased reflexes are of course, upper motor signs.
You do not mention any other such as spasticity or speech problems. Without some other upper-type symptoms and considering the slow progression, another look at MMN or other immune neuropathy certainly is worthwhile. My knowlege is limited, but I believe I have read of MMN without either conduction block or elevated antibodies. Four years is certainly slow progression. That is how MMN was discovered only about ten years ago.