Yeah Looking at the stat block its not great for any 4 man party. At levels 9/10 (even assuming they somehow get at him with full resources)
1)he can legendary a 6th level Banishment. Thats almost gauranteed to take out at least half the party unless countered.
2) he has a firball that will take almost anyone at those levels from full health to 0 hp instantly on a fail.
3)he has a single target pick someone to die ability that even on a save will probably still kill any injured target.
Its possible a party could nova hard enough to take him out but he has 2 abilities that can take out an almost full group by themselves, one of which he can use on a legendary.
I don't think you are correct and you are not considering the abilities 9th level characters bring to the table, especially things like Death Ward and Contingency.
Banishment will get 1 or 0 more often than it will get 2 or 3, considering saving throw proficiency, Paladin Aura, Invisibility, Lucky, Bardic Inspiration, Indomitable, Counterspell and a host of other abilities common among PCs at 9th level and it is also concentration and Soth only has a +5 Con save, so Banished PCs have a fair chance of being back in the fight before their turn even comes.
Again with the fire, it probably won't down PCs with d10 hit dice even on a failed save and a lot of the d8 characters will survive a failed save too. Your Rogues and Monks are usually going to take no damage at all, Wizards, Rangers and Sorcs are going to half the fire damage with Absorb Elements, and anyone who is actually downed gets back up with a mass healing word.
The word of death is potent, but it is once a day. On a fail a PC probably dies if they do not have Death Ward or resistance to necrotic.
If you catch the party when low on hps or after some other fights that exhausted their resources or if Soth has some mooks to support him, it would be a different story, but overall I think 1v4 would be a fairly easy for a 9th level party with all its resources.