The links below point to three different ways to install Oreo 8.1. Post any questions you have in the appropriate forum.The problem is I cant update my g4plus through OTA, it give me the message, "your devide is updated " but i havo android 7, is there another way to update to android 8.1 stock???
But first you might try a factory reset in the settings menu. This may trigger the official update to 8.1.
You'll need a USB cable connection between the phone and a computer with Android Platform Tools (fastboot and adb). There are several unofficial TWRP versions to chose from if you go that route. I suggest starting out with the official release from twrp.me.
Tip: If passing SafetyNet checks and Play Store certification matters to you, install build OPJ28.111-22 instead of the newer OPJ28.111-22-1 build. '22' passes where as '22-1' does not, although there's a workaround if you decide to root the phone with Magisk.
This OTA patch can be installed manually to upgrade Nougat to Oreo.
https://xdaforums.com/moto-g4-plus/how-to/guide-moto-g4-g4-oreo-soak-test-update-t3871136
Your build is NPJS25.93-14.7-8 which is older than NPJS25.93-14-18 mentioned for use with the OTA patch. I don't know if your Nougat build is new enough for this method to work but it doesn't hurt to ask in the forum.
Flashing the full fastboot ROM does not require an unlocked bootloader.
https://xdaforums.com/moto-g4-plus/how-to/stock-rom-npjs25-93-14-4-march-1-t3608138
There's also a TWRP flashable ROM. This is the easiest and fastest method but requires an unlocked bootloader and TWRP recovery.
https://xdaforums.com/moto-g4-plus/development/rom-oreo-8-1-0-soak-test-t3873367