Please correct me where I am wrong, and I am sure that I must be as all of this is new to me. My understanding is that 4130 is 2.5 times as strong as the same size piece in mild steel, so I shoould be able to build a frame out of it using the same O.D. tubing and wall thickness with an increase in strength of 2.5 times. Another option would be to use thinner walls in some areas and reduce weight as well. Does this sound logical to you?
Compare aluminum to steel by weight and aluminum is stronger. Compare aluminum to steel by size and steel is stronger. Aluminum is lighter (less dense) and therefore when compared pound for pound will be much larger than the equivalent weight piece of steel. This is intuitive by life experience if you think about it and you will find you already knew this. Squish a beer can and then squish a (steel) V-8 can.