If you cannot find a thick tree branch to hang a heavy bag from, you can build your own heavy bag stand for outdoor use out of heavy-duty lumber and lag bolts. Even if you have limited carpentry experience, you can put a simple rig together that will last a lifetime if you use all the right materials.
Build your frame at ground level first. Place one of your 4 x 4s on the sawhorses and use your reciprocal saw to sharpen one end to a point. Your bag will have to hang far enough away so you can move around it 360 degrees. Cut a 4-foot length off your other 10-foot length of 4 x 4 and line the flattest end up flush with the top of your foundation pole. Use metal framing brackets on the top and underside to secure these two pieces together. Drill two holes side-by-side through the back of the foundation pole 4 x 4, into the 4-foot length for the lag screws. Use a drill bit that is slightly smaller than the circumference of the screws, which should be at least 10-inches long and about as thick as a landscaping spike. Use a thick washer and screw in your bolts with a socket wrench or impact gun. Cut the remaining 4 x 4 length into two 3-foot pieces. Place these at a 45-degree angle and connect them to the foundation pole and the 4-foot length of 4 x 4 with long lag bolts. Pre-drill the holes in the foundation pole and two support beams first. Leave one side loosely connected before working on the other side. Secure the swivel to the end of the hanging beam and your stand is ready to stand up.
Dig a small one-foot hole and prepare some of the cement to fill it with. Get a helper and set up your stepladder next to the mounting hole. Have your sledgehammer handy and raise the stand up with the point in the centre of the hole. Once it is standing and in proper position, climb the stepladder and have your helper hand up the sledgehammer. As your helper holds the foundation beam in place, pound the top of the foundation beam with the sledgehammer so the point goes into the ground with at least five or six more inches of the beam. The structure should be able to stand completely on its own, and you shouldn't be able to pull it out of the ground if you have hammered it in deep enough. Pour your cement into the hole and let it dry once it is securely pounded into the ground.
Paint the whole stand with waterproofing stain or paint before you mount the bag on the swivel from the hanging length of chain. The bag should hang so the bottom is just higher than the knees of whoever will be using it. Have your helper hold the bag up while you connect the chains and hooks.