(I am guessing that you meant 10.
13.4
)
Downloads for macOS updates are found in the normal location -
https://support.apple.com/en_US/downloads
That gives you an updater that you can keep, if needed. The App Store updater download is a bit more challenging to keep.
So, one option is to download the macOS 10.13.4 combo updater from
here....
Or, if you have 10.13.3 right now, just get the 10.13.3 to 10.13.4
delta updater.
Usually, the delta updater is a lot smaller than the combo updater, but now, not much smaller.
Or, download the full installer for High Sierra, which you can get through the App Store, Featured page.
Make a bootable installer, and do that full reinstall to the most current version.