I considered the scenario you describe, and decided it is not a worry for a couple of reasons. I agree it is a theoretical concern, but not a practical one.
First, if I am drawing significantly different amperages from my 4 parallel pairs of batteries then I have very serious other problems that will kill the batteries quickly because of mismatched charge/discharge profiles that have nothing to do with the fuses.
If I am drawing power close to the maximum carrying capacity of the fuse, of course one will blow first. But then the power draw from the other three immediately jumps by 33%. For fuses already on the edge, that will surely cause a very fast failure. It is POSSIBLE, but very unlikely, that the power would be shut off before the failure cascades to the other batteries. In the case of the kind of short circuit I am actually trying to protect against, of course there is not a shut off option.
Finally, as fuses age, they can sometimes fail below their rated capacity. This is, I believe, the most likely way I could lose one fuse and not the others.
If one fuse was to fail, and I lost 25% of my battery capacity, I would very quickly notice that by the way I monitor my battery charge/discharge cycles. That is exactly the kind of problem I am always on the look out for because it could indicated a shorted cell in one of the batteries. A potentially dangerous situation I would very much like to know about if it happens.
The other problem I have found, is getting a fuse rated to interrupt 500 DC AMPS in one package gets rather difficult, and expensive.
Back Creek, Annapolis, MD