pmurphy

FYI, the reason I suggest digging a trench for your 4x4" is that the frame of your greenhouse will most likely create a "box" - meaning that if you attach it to the 4x4's it will raise the bottom of the door that same height and create a sill that you will have to step over. Not the biggest worry, you just have to remember to "step up" - which took me a couple of weeks and banged shins to remember with my first greenhouse (the second one sits at ground level). Your biggest issue will be if you are moving large or heavy things in or out, they will have to be lifted over the sill.
Pam