What does 4G63 mean?

These are engine model designators. 4G63 is the Mitsubishi model number for the Sirius engine lineup, which was built entirely by Mitsubishi. The 4G63 model number was used for 1G turbo, 1G non-turbo and 2G turbo engines; although the engines are somewhat different from each other, they retain the same basic design.

Please note that there are different 4G63 engines, although posters on the Talon Digest almost invariably mean the turbo 2.0L version. Generally speaking, components from one 4G63 can be fitted to another 4G63, because the basic components (head, block, etc) are the same.

420A is the model of the Chrysler-made 2G 2.0L non-turbo engine. Fundamentally different from the Mitsu 4G63 engines, the 420A represents a distinct shift in DSM evolution, as Chrysler took over engine duties on the non-turbo cars. Generally, components from 420A engines cannot be fitted to the 4G63 engines, or vice versa.

For more information on Mitsubishi motor codes and their breakdowns, see this guide from Road Race Engineering and this one from Turboclub.