The canonical reference for this is The OpenBSD FAQ - 5.1
The install4.8.iso in the 4.8 directory is the 4.8 before patches. So, if you want the patches, you need to install 4.8 then patch your system yourself.
The install48.iso in the snapshots directory is more than just the patches to the OS listed on the errata page, it's also everything new that is being developed as the system moves towards 4.9. Snapshots are just that "snapshots" of the code as it's moving towards the next release.
So, to answer your question, no. If you install using the install48.iso CD, you will not have a patched system, you will need to apply the patches yourself.
For information on applying these patches, see each individual patch.
You may also choose to follow the "stable" branch of OpenBSD, the reference is OpenBSD - Following stable, which includes these patches already.
In either case, you will have to have a checkout of the OpenBSD source.
There is no one-liner, or automated way to apply these patches.