4130 is considered a HTLA (Heat Treatable Low Alloy)steel. Filler metals matching the composition and properties of HTLA steels are available commercially, and AMS specification 6457A covers 4130. 4130 requires a heat treatment for maximum properties, but that seems out of the question considering this application pertains to a bike. Therefore, the use of a 309 stainless steel wire should work, a type 312 stainless steel, or a nickel alloy like a 625 stainless steel wire. Make sure the area of repair is very clean and free of any paint or grease. A MIG wire of probably no greater diameter than .035" diameter should serve you well. This type of wire can be purchased at your local welding supply distributor. The shielding gas, if using the stainless steel filler metal, can be straight argon or 98%argon/2%oxygen. The latter gas will give you very adequate shielding and very necessary arc stability. Good luck.