Considering 4" block for a 4.5' x 6.5' (interior) well pumphouse. 

Walls would rest on 8" thick footing, the bottom of which will be 32" below grade.  The interior floor of the pumphouse would be a floating 4" concrete slab over a backfill of crushed stone/sand and 6" below grade. 

Since the interior and exterior backfill will balance lateral forces, does anyone see a problem with using 4" below grade? 

The Logix tech guide shows 4" can be used for crawl space walls no taller than 4'-10".  This will be 5 courses of block or 6'-8", of which 2' will have interior and exterior backfill. 

I plan to use #4 rebar:  vertical @18",  horizontal @16".  Thanks Dave