A quarantine of only 4 weeks is for sure not enough. I had ich showing up 6 weeks into quarantine and then it went away by itself. Often fit and otherwise healthy fish get a handle on the infection by themselves, sometimes before you even notice it. So it is very easy to get ich into your tank without even noticing it.
It also may not show for a long time if ever. But if anything stresses or weakens the fish you may get a major outbreak.
To get your tank with 99.9% certainty ich free (I don't think 100% is truly possible unless you dump a bottle of bleach into your tank) you need to leave your tank fallow for an absolute minimum of 72 days. Though I would go full 11 weeks just to be sure.
During that time and after nothing wet (live rock, corals, crabs, shrimp,... can be added to the tank that hasn't gone through the same fallow time in a separate tank.
At the same time you process your fish through TTM. I personally would probably combine it with hyposalinity to be on the really safe side.
After that you hold them in a separate tank until the fallow period of the display tank is over.
From there on all new fish have to be handled through the same process plus some other medications to deal with other possible infections. After TTM and at least 4 weeks of observation without any findings you can add the new fish to your tank.
All inverts (with a few exceptions) and all wet items have to spend 11 weeks in a separate tank before going into the main tank.
That's the way you can get and keep your tank ich free.
I'm kind of torn if I want to do that or if I rather train new fish during their quarantine on the possible ich strains I have in my tank.