I also think there is some precedent for the convention of referring to these in the opposite order.
* level 1: PTE
* level 2: PDE
* level 2: PGDE/PDPTE
* level 4: PML4E

This way the meanings don't change as much depending on paging mode.  I think it would also reduce confusion between defining our own "layer 4" vs. the existing "level 4" that is part of PML4.