In a way, this is like the dotted-quad notation of IPv4, where each "level" of subchains is another dotted quad.   It might be more convenient to use 16 (0-9A-F) as the number of subchains per level.   

I would add that the difficulty of subchains should be vetted in the main chain, so that someone can choose an address that targets a level of difficulty they are comfortable with.   

It seems that subchains can safely be weaker than the main chain, because of the vetting done by the main chain, which acts, effectively, as a series of checkpoints.

Also, some addresses should be reserved for future use.   For example, an alternate hashing scheme could be added to address position 11 in a future where this is seen as desirable.  Again, since this is synched with the main chain, it benefits a great deal from the security the main chain provides, with the possiblity of a long-fork mitigated.   


0 - 10 full block subchains
11 - 15 reserved for future use