[Moto X4 Questions & Answers] taloras:
I've never touched the ROMS on the device beyond what it came with out of the box, it's using whatever the latest October OTA version is (there was an 11/1 OTA update which I saw a notification for but didn't apply).

fastboot getvar all gives me the following:

(bootloader) kernel: uefi
(bootloader) version-bootloader: MBM-3.0-uefi-2d29a27-181010
(bootloader) product: payton
(bootloader) board: payton
(bootloader) secure: yes
(bootloader) hwrev: P4
(bootloader) radio: NA
(bootloader) storage-type: eMMC
(bootloader) emmc: 32GB MICRON S0J97Y RV=08 PV=10 FV=4E5630304D483333
(bootloader) ufs: N/A
(bootloader) ram: 3GB MICRON LP4 DIE=12Gb M5=FF M6=02 M7=10 M8=0C
(bootloader) cpu: SDM630 1.0 (2)
(bootloader) serialno: ZY2257LBZP
(bootloader) cid: 0x0032
(bootloader) channelid: 0x8d
(bootloader) uid: 662200ED
(bootloader) securestate: flashing_locked
(bootloader) verity-state: logging (0)
(bootloader) iswarrantyvoid: yes
(bootloader) max-download-size: 536870912
(bootloader) reason: Volume down key pressed
(bootloader) imei: xxxxxxxxxxxx
(bootloader) meid:
(bootloader) date: 09-19-2018
(bootloader) sku: XT1900-1
(bootloader) carrier_sku: XT1900-1
(bootloader) battid: SNN5994A
(bootloader) battery-voltage: 4284
(bootloader) iccid:
(bootloader) cust_md5:
(bootloader) max-sparse-size: 268435456
(bootloader) ro.build.fingerprint[0]: motorola/payton/payton:8.1.0/OPWS2
(bootloader) ro.build.fingerprint[1]: 8.46-21-15/ec37:user/release-keys
(bootloader) poweroffalarm: 0
(bootloader) ro.build.version.full[0]: Blur_Version.28.261.23.payton.ret
(bootloader) ro.build.version.full[1]: ail.en.US
(bootloader) ro.build.version.qcom: LA.UM.6.4.r1-06900-8x98.0
(bootloader) version-baseband: M660_7042.27.01.74.02R PAYTON_NA_CUST
(bootloader) kernel.version[0]: Linux version 4.4.78-perf-gf993b65 (huds
(bootloader) kernel.version[1]: oncm@ilclbld33) (gcc version 4.9.x 20150
(bootloader) kernel.version[2]: 123 (prerelease) (GCC) ) #1 SMP PREEMPT
(bootloader) kernel.version[3]: Wed Oct 10 02:52:24 CDT 2018
(bootloader) git:abl: MBM-3.0-uefi-2d29a27-181010
(bootloader) git:xbl: MBM-3.0-uefi-455afa3-181010
(bootloader) git:pmic: MBM-3.0-uefi-455afa3-181010
(bootloader) git:rpm: MBM-3.0-uefi-f14962c-181010
(bootloader) git:tz: MBM-3.0-uefi-9a58d2e-dirty-181010
(bootloader) git:hyp: MBM-3.0-uefi-9a58d2e-dirty-181010
(bootloader) git:devcfg: MBM-3.0-uefi-9a58d2e-dirty-181010
(bootloader) git:cmnlib: MBM-3.0-uefi-9a58d2e-dirty-181010
(bootloader) git:cmnlib64: MBM-3.0-uefi-9a58d2e-dirty-181010
(bootloader) git:keymaster: MBM-3.0-uefi-9a58d2e-dirty-181010
(bootloader) git:prov: MBM-3.0-uefi-9a58d2e-dirty-181010
(bootloader) git:storsec: MBM-3.0-uefi-9a58d2e-dirty-181010
(bootloader) frp-state: protected (77)
(bootloader) ro.carrier: retus
(bootloader) current-slot: _a
(bootloader) running-bl-slot: _a/_a
(bootloader) running-boot-lun: 0
(bootloader) slot-suffixes: _a,_b
(bootloader) slot-count: 2
(bootloader) slot-successful:_a: no
(bootloader) slot-successful:_b: no
(bootloader) slot-unbootable:_a: no
(bootloader) slot-unbootable:_b: no
(bootloader) slot-retry-count:_a: 5
(bootloader) slot-retry-count:_b: 0
all: listed above

28.261.23 and 8.46-21-15 look sort of like build numbers; that plus a pretty recent Oct 10th kernel build date leads me to think I'm looking for a 28.46-21-15 or later firmware download, which I'll try to track down. Know of anywhere I could find firmwares that recent?

I have an older desktop with a bunch of usb 2.0 ports on it, I'll give that a whirl too.