Can I use this to secure a 4x4 post to a cement deck block?

Thank you for the question. Not knowing the details of the project, using Loctite PL 500 Landscape Block Adhesive in this capacity may require permanent mechanical attachment depending upon the function of the post. In general construction adhesives of any type should not be considered as the sole means of attachment for structural purposes or where safety is concerned. In this situation there are no guarantees.