For a small building like this, 4X6', you really don't need to dig in a below frostline foundation for the posts. Just put it on skids and level it. For my coop, 8X16', it is supported at six points on concrete blocks, sitting on shallow holes filled with gravel. Been sitting there for years like that with no problems. If a corner starts to settle, I'll just jack it up and shim. I mean, we are talking about a chicken coop here, not the TajMahal.