1020 is sometimes hard to find. 1018 is much more plentiful and specs out nearly the same as 1020. I use to like 1020 because it cuts easier. The aircraft I use to work on you could use 1018 or 1020 interchangeably. 1025 called for different applications but . . . .
The problem with 4130 is that most people don't like working with tubing thin enough to really take advantage of its greater strength. It's trickier to weld and especially trickier to bend.