This is a terrible idea for a lot of different reasons, the biggest one being that PSO2 simply isn't built for this kind of scaling.
The simplest problem here would be in Expeditions that allow for more than one party at a time. You're asking for enemies to scale to four times their health if you run with a four man party, but what happens when your party runs into another party, such as in an Ultimate Quest? Does that mean that suddenly the monster you're attacking as turned into a giant hulking damage sponge because eight other people are in the field with you at the same time?
What if those people in the field are running with NPCs, does the scaling affect you then? What if you're the odd man out in an Expedition or Ultimatae Quest in an instance that's at capacity. Does that mean while you're running around solo, you're fighting monster with twelve times the HP, just because you had the unforunate luck to zone into a full instance?
Furthermore, let's say you decide that the scaling is only on a per party basis, so if your party tags an enemy first, it scales to 4X it's HP, what happens then when another party comes in and joins the fun, such as when a field boss spawns in an Expedition. Now the enemy is still getting McMurdered because it's scaling is capped at 4X, but you have 12 people wailing on it. How would that work in an Ultimate Quest? Do we turn everything there into massive damage sponges, resulting in farming becoming a mind numbingly slow chore, or does it suddenly become the new meta to have a solo player tag an enemy to keep it's scaling to 1X, while everyone else in the instance murders it.
What about party beacons? How does scaling work in the field when people can join you dynamically? How does this affect other people in the instance?
Going beyond Expeditions and Ultimates, how does this apply to Urgent Quests, which are designed to be run by 12-man raids?
What I'm getting at is that PSO2 simply isn't built with your idea in mind. The game would completely fall apart at the seams if you tried to implement it. Yes, World of Warcraft does have dynamic health scaling in place out in the open world and raids (and a very small change in mechanics in Raids depending on how many people you bring with you), but this was done for two reasons - the first being that people were struggling to get tags on quest mobs and rares in the open world when a single player could literally one shot an enemy, and in raids it was done so that more casual groups could run with fewer or more people without having to bench someone.
But even WoW turns this off when it comes to Mythic Raids, where the content is locked to twenty-man raid parties, simply because Blizzard knew it's impossible to tightly tune extremely hard, cutting edge content when player scaling is a thing.