People have been known to use this base with 4" x 6" posts in the past, however, the intended design and recommendation is for 3.5" - 4" x 3.5"- 4" posts. The engineering and testing documents for this item were done on 3.5" - 4" posts, so we are unable to provide a recommendation on stability since a 4" x 6" post was not tested with this post base. Thank you for your inquiry.