Messages in iCloud
We all know that you can check your email on your phone, your computer, or your iPad. And deleting an email from one device will delete the email from all of your devices. The system works because all of your devices talk to a single server to handle your email. iOS 11.4 brings this same feature to your text messages.
Before iOS 11.4, if you deleted a message thread on one device it would still exist on other devices. And while new text messages would normally show up on all devices, sometimes they would appear on one device but not another one. And sometimes messages would display out of order on one device. In iOS 11.4, once you turn on Messages in iCloud, iCloud acts as a central hub for all of your messages (both SMS text messages and iMessage messages) so that all of your devices can stay in sync. And iMessage is encrypted end-to-end for your privacy.
To enable Messages in iCloud, open the Settings app, tap your name at the top, then tap iCloud and turn on Messages.
At least, that is how it is supposed to work. Last night, the Messages app in iOS 11.4 worked great for me on my iPhone, but on my iPad the app seemed to get caught on the "Signing in..." screen, where it has been stuck for many hours. I tried signing out of my iCloud account on my iPad and signing back in again, but that didn't fix the problem. I haven't yet seen any other reports of something similar, so perhaps this was a hiccup unique to my iPad Pro. I'll update this post when I figure out how to get this working.
[UPDATE 6/6/2018: It took about five days, but eventually the app started working correctly on my iPad. I presume that there was some sort of bug on the Apple server that affected some devices such as mine, but now all is working great.]
Note that even though the messages are stored in iCloud, that doesn't mean that you can see them at the iCloud.com website. That website does give you access to other items synced via iCloud such as mail, contact, photos, etc. But for now, at least, there is no Messages app on the iCloud website.
Also note the keeping your messages one the iCloud server uses up some of your iCloud data space. If you are not paying Apple for additional iCloud space and if you have lots of pictures and videos in your messages, you might not have enough space on iCloud.
If you own an Apple HomePod, iOS 11.4 is an important update for you. Especially if you own multiple HomePods. With AirPlay 2, you can put two HomePods in one room for richer, stereo sound. Or you can place them in different rooms and the music will stay in sync as you travel from room to room.
If you own a smart speaker from another company, it may also support AirPlay 2. Apple has a page on its website listing dozens of devices from manufacturers like Sonos, Marantz and Devon that will also work with AirPlay 2.
Fixes and Security
Virtually every iOS update fixes various bugs and improves security in various ways. iOS 11.4 fixes issues with CarPlay in which audio can be distorted. I don't yet know exactly what this means; I've noticed that CarPlay in iOS 11.3 would occasionally cause some popping noises for me, and perhaps this fixes this. iOS 11.4 also fixes some issues that arose when accessing certain Google files in Safari including Google Drive, Good Docs, and Gmail. Apple also fixed a bug that could cause Messages to crash if certain characters were sent in a text message. And Apple will soon update this page with information on the security improvements in iOS 11.4.