I don't think there are 4130 subframes, and if there are you don't want them.
Chrome moly when welded needs to be stress relieved or the weld area will be very weak. It can be done, but be sure your welder knows what he's doing. Also, you'll have quite a bit of galvanic corrosion anytime the car gets wet from the two different metals in contact.
Stick with chrome moly for cages, etc where it's inside the car or not welded in