The difference between an 8 byte salt and a 4 byte salt isn't 4 bytes, and it isn't a factor of 2.  It is a factor of over 4 billion.  It is the difference between "Got a minute?" and "Sometime after the end of the universe".

In reality, it is probably not that big of a deal.  Precomputing AES tables isn't a trivial task, and even 4 or 5 bits of salt is probably enough to make it impractical for well beyond the potential lifetime of one of these protected keys.

But cryptosystems are designed with defense in depth for very good reasons.  When a weakness in one part is discovered, it is usually not a catastrophe because we have added redundant security measures.

So, the question is:  Is saving a few keystrokes, maybe several times per year for maybe a couple billion users, worth the slight risk that an AES break won't be "phew, gotta upgrade", but instead be "All your addresses are belong to us"?  I would vote for the extra keystrokes, but that might just be me.

By the way, this is a nifty idea.  One thought that came to mind was embedding metadata into the key itself using DER.  Since these keys are not usable on the network, they should maybe not be assigned a key version prefix.