Um. I'll preface this post with the statement that I don't plan on participating in this RP at the moment, but I'm willing to drop my two cents in to help out a fellow roleplayer since I do enjoy the Rising of the Shield Hero franchise.

If you want to run the setting "as is" (i.e. Naofumi etc don't exist but the characters of the RP do), the best way to run this in my opinion would probably be as a Shield Hero + Party roleplay with the Shield Hero as the GMPC. The Shield Hero is more reliant on their party to take any action (read: players are significant) and is capable of defending them (read: bailing them out) effectively with their weapon. Plus the Shield Hero has all the lore baggage that the other heroes don't have, which makes for interesting potential plotlines. 

I would recommend against a Four Heroes + Party RP since that invites spotlight stealing, and requires a concession for the "all heroes must journey separately" rule. Or like, four players max unless people really want to play party members which, to be fair, is not really unlikely.