I think the 4200 is slightly faster, and is going to cost less. It is almost like a replacement I think.
That is why they didn't release it earlier, because it would of made buying the ti500 a very bad choice.
The 4200 is an "MX" card, really low end crippleware... I'd go for the Ti500.
The 420 is an mx the 4200 is a regular g4.