I can't think of any good reason to do this (and wouldn't recommend it), but if you had to satisfy this requirement of a 4MB block size you could probably create a VMFS3 volume and then upgrade it to VMFS5 to retain the 4MB block size. New VMFS5 volumes use a unified 1MB block size.
Take note of the limitations;
VMware KB: Block size limitations of a VMFS datastore
Cheers,
Jon