In the tech world, upgrades are normally a good thing. On iOS, it means bug fixes and security patches that protect your iPhone and keep it up with the times. In the jailbreak world, however, upgrades can mean certain death, as we are reminded today when Apple stopped signing iOS 11.4. That means if you upgrade to 11.4.1, you will no longer be able to jailbreak your iPhone.
When Apple stops "signing" a software, iTunes won't recognize it, nor will it install it on your iPhone. That usually isn't an issue, as you typically don't need to revert to a past version, but jailbreakers need to stay one or two steps behind Apple since it takes some time to exploit iOS.
Right now, jailbreak developers working on iOS 11 have provided us with a semi-tethered jailbreak for iOS 11.2 through iOS 11.4 beta 3. So, if you upgrade to 11.4.1, you won't be able to go back, since iOS 11.4 and below are no longer being signed by Apple, meaning you've lost your chance to jailbreak until someone figures it out for 11.4.1.
There are ways around this issue, but it takes some foresight and preplanning. You can save your SHSH2 blobs to revert your iPhone to iOS 11.4, even when Apple stops signing it. This method "tricks" iTunes into thinking 11.4 is still signed, allowing you to downgrade back to a version of iOS compatible with the most stable jailbreak available today. If you are still running iOS 11.4, think about saving your SHSH2 blobs as jailbreak insurance.
If you've already upgraded to 11.4.1 without saving your blobs, all hope isn't lost. While you cannot jailbreak your iPhone as it currently stands, devs might find an exploit to jailbreak iOS 11.4.1. However, it's possible this version will be skipped entirely for iOS 12, as Apple's next big software release — currently in beta testing — is right around the corner. In preparation, however, you might want to save those blobs now, before Apple stops signing 11.4.1.