Ajax with Ten Hag in 4 full seasons: 1st place 4 times, 2 cup wins, twice past group stage in the CL.
Ajax hadn't won the league for 4 seasons before ETH. Ajax hadn't reached the knockout stage of the CL since 05/06. Ajax hadn't won the league and cup double since 2001/02. Ajax hadn't won the league and reached the knockout stage of the CL since LvG was there in the mid 90's. Ajax have only ever shown a similar level of dominance in the league with points, goals, performances, success with their greatest ever managers (Cruyff, Michels, etc). Ajax have averaged 3 league goals per game since he joined, or an average of over 100 goals per season for them, over a 4 year period. They passed 100 goals in the league in 09/10 with Suarez before he went to Liverpool, but the last time they regularly had a team up there was LvG's great teams and then the early 80's team.
Its really not a list comparable to other managers who achieved success and then failed elsewhere. ETH has shown dominance across ever competition, playing a great style of football, has them performing as much more than the sum of their parts, going toe to toe and outplaying pretty much any team they play even if they are a much wealthier side, and all this every year despite constantly losing his best players, so needing to adapt. They still play with the same ideas, just with adjustments to different set of players. They'll dominate Madrid and Juve, lose De Ligt and De Jong, and then go out and outplay Chelsea away the following year and only get unlucky with referee decisions. He's a fantastic coach, but of course there is always unknowns in how he will adapt to the prem. We'll see. But it's the best possible appointment.