If that were the case, the arguments favoring evolution would be much more clear-cut. Unfortunately, that is far from the case.
Suppose an individual of some species has a random mutation on one of its #4 chromosomes which causes the development of photosensitive cells. This trait will be present in half of this individual's offspring, 1/4 of its offspring's offspring, etc. Unless the gene improves the reproductive success rate of those individuals possessing it, it's unlikely to ever become widespread.
Also, I'm curious: are there any pairs of animal species with different numbers of chromosomes which have ever been inter-bred so as to yield fertile offspring which could in turn yield further offspring, etc.? If not, that would suggest that the only way a species could evolve into another with a different number of chromosomes would be to have the same mutation simultaneously hit enough individuals to produce a sufficient breeding stock of the new species. I have seen no mechanism proposed by which that could take place.