yes you can. some separation features available in magento by default, some you can adjust with extensions, even separated filesystem and its own database, but.
this is all about security and maintenance complexity and as a result any issues affects reputation.
it is much easier to run 4 completely individual installations.
this is how managed hosting works. automate, snapshot, template.
control everything and manage with automation. as you have the same OS image for every container / instance you can easily track changes and updates with for example ansible or simple scripting from control unit. server configs are the same, customer only have their includes to adjust some parameters
magento versions and modules you can track the same with simple scan across all of your clients. they must have magento core and default code intact. also you can see how much they have overrides with simple scan (like for example magento debug extension do to find core overrides and conflicts)
for native composer modules you just upgrade with single command, with local modules you must keep your own repo to push with composer, etc.