I want to make a backplate for a 5" Bison 4 independent jaw chuck (the
one recently on sale at Enco). I've read that cast iron is preferred
for this, but since my spindle is not threaded (Wabeco D3000E lathe) I
don't have to worry about galling threads. Online Metals will sell me a
4" x 1" disk of 4130 alloy steel for $17. That is cheaper than any of
the rough-cast backplates I can buy and I won't have to make a pile of
iron dust. It appears that 4030 is the only material they sell in a 4"
diameter. So will 4130 work acceptably for machining a backplate or is
this a bad idea?

The chuck really only needs a 4" back "plug" since the recess on the
back of the chuck is only 3" in diameter and the mounting holes are
within that circle. The lathe spindle plate is under 4"