Supposedly, the director's original plan was to make GS4 (Apollo Justice: AA) exist in a mostly-separate storyline with no/few characters from the original GBA games making a reappearance (I'm theorizing that only the case 1-5 characters such as Ema re-appeared in the original plan, along with Gumshoe), but the higher-ups at Capcom were afraid that the series' fans wouldn't accept the new character. So to appease Capcom, the director replaced the "fallen veteran attorney" character with Phoenix mid-development. I don't remember where I read this (Probably a court-records topic), but it seems like a reasonable conclusion that the original plan for GS4 didn't include the PW trilogy characters with Case 5 serving as the first bridge between the PW and AJ storylines, especially when you consider that none of the non-Case 5 characters appear in this game.