The target model just has little things target shooters typically want. It's a little heavier, grip angle is different than the 22/45. It also has adjustable sights unlike the standard model which has fixed sights. It's also drilled/tapped for a scope base and comes with one in the box.

The target has a Bull barrel

Correction, they no longer include a scope base with them, but they are drilled/tapped.

The Mark 4 Target generally has a bull barrel over the standard barrel of the regular Mark 4 pistol. It also is drilled and tapped with scope rail for mounting a scope or red dot sight..