Ubuntu 18.04.4 with all upgrades applied becomes 18.04.5.
The [18.04].4 or [18.04].5 shows the upgrade level, and is more significant if using the HWE (hardware enablement) kernel as 18.04.4 used the now EOL 5.3 kernel, where as 18.04.5 uses the 5.4 kernel; if using the GA (general) kernel they'll both be 4.15.
To go backwards means reverting security fixes and is not a wise move.
Apt/deb package tools won't do that anyway; offering only automatic for newer/upgraded packages, so a backup restoration is easiest way to go backwards, but that is still unwise.
Ubuntu 18.04.4 with all upgrades applied becomes 18.04.5.
The [18.04].4 or [18.04].5 shows the upgrade level, and is more significant if using the HWE (hardware enablement) kernel as 18.04.4 used the now EOL 5.3 kernel, where as 18.04.5 uses the 5.4 kernel; if using the GA (general) kernel they'll both be 4.15.
To go backwards means reverting security fixes and is not a wise move.
Apt/deb package tools won't do that anyway; offering only automatic for newer/upgraded packages, so a backup restoration is easiest way to go backwards, but that is still unwise.