4130 is fine for MIG welding, however there are a number of reasons why you might not want to MIG weld your frame.
The first is that MIG tends to produce pretty fat weld beads, not exactly neat. OK so you could grind or file it down.
The second is that when you're welding a crack you need accuracy, this can be difficult particularly if the crack does not run straight. MIG, particularly with hobbyist plant, tends to be difficult since it's hard to see the weld puddle because it will be obscured by that big fat gas nozzle.
The third and final reason could be the show stopper. Remember MIG was originally designed for automation. In this situation wire feed and current are pre set for known material thickness and weld speed. When converted to hand held use the same controls still apply. This is great where you're welding known workpiece thickness, particularly since you can practice on scrap steel first to get your settings dead right. You don't have that luxury on your frame. Do you know how thick the tube wall is at the break? How will you know if you've got full penetration? (Ooooerrr!) Failure to get full penetration could lead to the weld failing. OTOH too much current and you could blow a hole in the workpiece.
I would tend to go for TIG since you have so much more control. Some prefer using gas because the plant is cheaper, but TIG gives better heat control for thin material. MIG doesn't do delicate, TIG OTOH could be used to weld a razor blade to an engine block edge on.
The fact you are asking about MIG welding suggests you are looking to do a home repair. Are you sure you can do a good job given my questions above?
You might be better off taking it to a frame builder for assessment. Any reasonable frame builder would give you a free quote, and indeed let you know if it is repairable at all. Depending on the nature of the fracture it may no be worth repair. Have you, for example, cleaned up around the fracture to see if it's a clean break? Is there much corrosion in the area? Like my welding tutor said, "you can't weld rust".
Having said all that I do know a man who once affected a successful frame repair with a basic MMA "buzz box" welder. He found himself about 400 miles from home in Scotland with a chainstay cracked near the BB. Rather than cut his tour short he decided to repair the bike, but he couldn't find a frame builder in the highlands. However he did find a farmer who had an old buzz box he used for repairing his farm machinery, trailers and the like. It wasn't neat or tidy (he was using contact rod) but it did the job. OK so he is a professional, but even so it goes to show it's possible.
