Sorry for late reply- If you don't wish to cut up bike you can buy tubes of the correct size which may be a bit cheaper than buying a reamer for non-production building. I purchased BB tubes from this site:

As to 4130 the preheat will assist with welding via Tig or using Gas welding. But post cooling is not an issue with our gauge material. Aluminum has those issues. Not 4130. Just use ER70S-2 filler from what I have read and been recommend here. Here is more reading on using 4130: Biggest hurtle is going to be sure you have a really good fit up. With 4130 you will want better fit than with mild steel. I believe because you are using the ER70S-2 filler.

While the 4130 is not truly lighter than 4130- due to strength of it you can use thinner wall tubing to achieve lighter frames for the same strength. So instead of .065 material we can use .049 material and have the same strength as the .065 in mild steel. The .065 4130 and .065 mild steel weight the same. But due to its extended strength we can use the ,049 to achieve a slightly lighted frame. However the wheels and other non-frame components add more weight.

As to reaming tubing you can do that but I would caution not to make it too thin. I reamed 4130 tubing to create a head tube similar to : I am not planning on using bicycle parts from used bikes for my folding project. The project got put aside for other issue and the momentum has not got started again.

Hope that helps.