Using wood glue and 1 1/4" pocket hole screws, attach the frame you just built (36" x 11") to the 4x4 frame posts. You will use the pocket holes you drilled in the 36" boards. The frame will sit on the edge of the grooves, not inside. See plans for details. The frame should sit flush with the tops of your 4x4's. This will make the bottom 4" off the ground.
Building a coffee table with hairpin legs is fairly easy, especially if you choose a simple design for the top. There are several options to choose from in that case. One is to build a simple frame which looks something like this. That way the coffee table would look pretty, would be easy to build and would also include a storage compartment which is always useful.
Please read through the entire plan and all comments before beginning this project. It is also advisable to review the Getting Started Section. Take all necessary precautions to build safely and smartly. Work on a clean level surface, free of imperfections or debris. Always use straight boards. Check for square after each step. Always predrill holes before attaching with screws. Use glue with finish nails for a stronger hold. Wipe excess glue off bare wood for stained projects, as dried glue will not take stain. Be safe, have fun, and ask for help if you need it. Good luck!