First of all, welcome to the forum! There are a lot of people in here who can share a wealth of welding knowledge.
Second, I am sure we would all like to know what code you're welding to. For instance, I can answer D1.1 questions, but I know next to nothing about ASME. Lots of other guys know ASME pretty well.
From my experiences with 4130, your choice of filler isn't the best. The ER70S filler is great for low carbon steels, like A36, but once you start going higher with the carbon and add the other ingredients that make up a high strength low alloy steel, you run the risk of making an extremely brittle weld along the toes, where the filler dilutes with the base metal. That's likely 90% of your problem right there. Try a filler made for HSLA steels, like ER80S or ER90S.
The rest of it probably has to do with the heat treatment. In this case (in my opinion), the heat treat at 600F isn't really doing anything. What is the purpose for the heat treat? Is it just a stress relieve operation, or for hardening? What is the soak time? Do you control the cooling, or just take the heat off and allow it to come down to room temperature? Another consideration: did you preheat the joint?
I hope I have helped a little, but I am curious about the heat treatment parts.
It looks like the force tore the base metal on the vertical tube. The fracture probably started at the top at the toe of the weld. My guess is that the vertical tube is too thin to restrain the bending moment without flexing through the thickness of the tube. Just based on dead weight, there is a 900 lb/ft moment force at the connection. If the 300 lb. weight is being dropped, then there is an impact load that is much higher than 900 lb/ft. You might try increasing the wall thickness of the vertical tube and/or adding a gusset or brace on top.