Glue a large bolt or allthread rod into one end of the tube. I'd use some sort of epoxy resin or low-temperature thermoplastic. It will be helpful to rig up some jig to hold the bolt in position while the glue sets.
Then you can drill the base and either tap the hole or use a washer and locknut on the underside (if application and space permit, otherwise add feet at corners to make space)
You could also tackle this the other way around, drill the base, countersink the hole, glue a nut into the tube and use a countersunk bolt through the base into the tube.
Ditto pilot-drilled wooden dowel, glue and large woodscrew or coachscrew depending on environmental factors and anticipated mechanical loading or abuse.
Four foot gives a lot of mechanical leverage. Don't expect this to be indestructible.