The only reason to wire the 4-way separately would be if any of the other controls might get bumped or jostled during vigorous game play. In other words, if you are playing a 4-way game and your elbow accidentally bumps one of your 8-ways, you may get unintended inputs. Or if you have small children who like to grab a control while you are playing.

I would say this is a risk with almost any multi-control CP but one that almost everyone accepts. For most people it is unlikely to be an issue and thus we usually wire them into the controller in parallel.
There are other ways you could isolate the controls as well if need be....say a two position switch that selects between the grounds for each joystick. That might be a way to go if you don't want to use up precious inputs with a dedicated 4-way.