One of these things?
Or, if you had some very large angle iron. Put some bolts or steel rods
under the surface to hold-down, and 3" lag bolts up top..
Or, a pair of angle irons cross bolted together (with a 4" spacing)
under the concrete, so the 4x4s would fit between them, up top.
Use 3" lag bolts or drill the 4x4s and use 3/8" or 1/2" carriage bolts with washers & nuts.