4 weeks is really on the shorter side of things for most people who are serious about quarantine... 8-10 weeks is completely reasonable to accommodate observation and treatment periods (for fish).
As far as coral/inverts go, it really depends on what you are trying to keep out. If you are on the lookout for coral hitchhikers and pests, ie aiptasia/flatworms/redbug/etc, 4 weeks might do the trick. If you are concerned about ich coming in on corals (which is very possible), you will essentially want to fallow the coral for the 8-10 week period in a separate system/qt area with no fish. While corals cannot "get ich" (as it is a fish-only disease), parts of the ich life cycle take place in the water column and on substrate, rock, or even on coral plugs/skeletons. This theory applies to other fish parasites as well.