To be perfectly honest, I think using the "monster" type NPC stats in a party is probably not supported by the RAW in general. If these were temporary allies who were going to hang around for a session or two, you could easily fudge it, but if they're meant to replace PCs because of a player shortage, you're going to be far better off creating the "stand-in" NPCs as if they were PCs.

There may be a lot of ways you could simulate, run some kind of statistical regression or whatever to estimate what level equivalent those NPC stats should be, but none of that is going to be less work than rolling up four 1st-level characters to be the co-stars in your one player's story.

To be perfectly honest, I think using the "monster" type NPC stats in a party is probably not supported by the RAW in general. If these were temporary allies who were going to hang around for a session or two, you could easily fudge it, but if they're meant to replace PCs because of a player shortage, you're going to be far better off creating the "stand-in" NPCs as if they were PCs.

The two biggest components to a "monster's" challenge are its hit points and damage output per round, and to keep that up, "monster" stat blocks get things like multiattack and gang-up bonuses at pretty low level (like the Thug, with its challenge of 1/2). Stuff like that is what's going to make it particularly hard to approximate, because PC levels are made up of things like the breadth and depth of their abilities and their capacity to handle noncombat tasks on top of combat, and that's what your player is really going to be missing out on.

There may be a lot of ways you could simulate, run some kind of statistical regression or whatever to estimate what level equivalent those NPC stats should be, but none of that is going to be less work than rolling up four 1st-level characters to be the co-stars in your one player's story.