I am building a leantoo 12x20 for horses, I want to use 4x4's for posts, it will be 12' in front and 8' in rear, will use 2x6 rafters with osb and shingles, will use hangers on the top ban of 2x8's, If I go 4 foot on center with the posts, and 4' in ground with corners cemented, will 4x4's be ok, side walls will be 2x4 and 2x6 purlins, sheeted with half inch plywood. The front will have an 8' opening, 8' high, I want to use 4x4's because of access and I can set them myself, any help or suggestions will be apperciated.