Zini, like others, discovered that if the "if the Raspberry Pi 4 outputs HDMI at a resolution of 2,560 x 1,440, the Wi-Fi stops working".
It doesn't happen at lower resolutions like Full HD or 1,920 x 1,080. Zini also notes that many users have reported that Wi-Fi works fine with 4K resolutions.
The glitch is important for Zini who is working on a project to use the Raspberry Pi 4 connected to a display as a digital signage system for places like museums or schools.
Dozens of Raspberry Pi 4 users have reported similar display output Wi-Fi problems on the Raspberry Pi forum. However, the reports are inconsistent and there are lots of variations in setups, such as the length and type of HDMI cable, power supplies, and other connected devices.
The exact cause remains a mystery, but a Raspberry Pi Foundation engineer speculated that a possible cause is radio frequency leaking from HDMI cables and causing interference with Wi-Fi signals. Some users have noticed a different impact on Wi-Fi, depending on which of the two HDMI ports are being used.
Zini says he reproduced the Wi-Fi glitch on three different Raspberry Pi devices, using different HDMI cables and adapters, and multiple power supplies.
Raspberry Pi Foundation boss Eben Upton isn't sure what's causing Wi-Fi to stop working, but he also guessed it was interference from "poorly shielded" HDMI cables, as Android Authority reports.
Upton today told ZDNet that the foundation is planning to issue a firmware update in the next week or so that should address the issue. He also offered a workaround for those who want to use a 2,560 x 1,440 resolution.
"We're aware of the report, and have managed to reproduce the issue with one particular access point and (cheap) cable. The original reporter seems to have been using the official cable, so that's a bit of a mystery," Upton said in an email.
"We're likely to ship a firmware update to address the issue in the next week or so. In the meantime, anyone who wants to use the 2,560 x 1,440 resolution, and who is experiencing this issue, should adjust their router to use 2.4GHz channel 4 or above, or the 5GHz band."
If your Raspberry Pi 4 is running a little hot, users can get it running cooler simply by positioning it vertically with the GPIO header at the bottom and the power and HDMI ports at the top.
"Simply moving Raspberry Pi 4 into a vertical orientation has an immediate impact: the SoC idles around 2°C lower than the previous best and heats a lot more slowly – allowing it to run the synthetic workload for longer without throttling and maintain a dramatically improved average clock speed," explains Gareth Halfacree from the The MagPi magazine.
"There are several factors at work: having the components oriented vertically improves convection, allowing the surrounding air to draw the heat away more quickly, while lifting the rear of the board from a heat-insulating desk surface dramatically increases the available surface area for cooling."